As I noted on The Warrenverse, the Warren characters--including but not limited to their three big guns, Vampirella, the Rook, and Pantha--did indeed interact with each other a good number of times, albeit not nearly as extensively as the Marvel and DC characters did, so it should be relatively easy to get a crossover list for them.
The ultimate Warren crossover was the book-length story for "Vampirella and the Time Force" in EERIE #130, which was a major cross-time crossover between Vampi, Pantha, the Rook, their supporting casts, and the following Warren characters who all once had their own series: Exterminator One, Demian Hunter, Derrick Schreck, Child, Dax the Warrior, and the Pie. A much briefer crossover of the above characters occurred with the following Warren characters at a later point in the story, all of whom also once had their own series by Warren, thus establishing their presence in a shared universe: Darklon the Mystic, Karas Hunter, Myron Maximilian Mac Tavish, the Spook, and Coffin.
My index covering the above story in detail is here.
Here are some additional major crossovers of note in the Warrenverse:
Warren's version of Dracula--whom several of us Wold Newton researchers consider to be the soul-clone we call Dracula-Mordante--crossed over with Vampirella in several stories during the early '70s (and continued doing so during her Harris days, and now into her current days at Dynamite); he once crossed over with Sherlock Holmes (who had a short-lived series with Warren during its final days); and he had two separate series of his own in Warren Comics. The index on Warren's Dracula (i.e., Dracula-Mordante) that covers all of the above in detail is here.
It has been shown in Warren Comics that a prominent alternate future timeline that I call the Hunter Timeline, which some Warren fans have considered to be the backbone of Warren's shared universe, later revealed that Derrick Schreck, Demian Hunter, Karas Hunter, and Darklon the Mystic (all of whom had their own separate series published by Warren) all existed in the same alternate future timeline, albeit their various series took place at different points in time on that future track.
Schreck shared a major crossover with Demian Hunter in the last two entries of the latter's series when Schreck was an elderly man; Karas Hunter was Demian Hunter's successor to the name and mission (they weren't blood related), who was directly inspired by his career; and Darklon the Mystic was later revealed to be Demian Hunter's descendant in a self-contained crossover story that was published by Warren after both of their series had ended; and perhaps strangest of all, Darklon actually went on to cross over with Adam Warlock of the Marvel Universe when Jim Starlin, the creator of Darklon and the craftsman who re-interpreted Jack Kirby's late '60s creation Him as Adam Warlock in the early '70s, was penning WARLOCK CHRONICLES for Marvel in the '90s (though now referring to Darklon as 'Darklore'). Further, a line of the Exterminator cyborg assassins existed on the Hunter Timeline, and the last of the Exterminator mecha-soldiers was a major supporting character in Karas Hunter's series (i.e., "Hunter 2"), as well as in a later self-contained Karas Hunter story.
The full skinny on all of this can be found in my indexes to the Schreck, Hunter, Hunter 2, and Darklon the Mystic series (just click on the respective hyperlinks).
Importantly, the Hunter/Darklon the Mystic crossover tale from EERIE #120 (entitled, "Ashes to Ashes"), not only featured a quick cameo appearance of a member of the alien Cassiopeans, the extraterrestrial race to whom the Pie belonged, but also a rather significant appearance by Quarb, the immortal alleged progenitor of the human species, who was a recurring character of note in the Rook's series.
Finally, in regards to crossovers on the Hunter Timeline, the true origin of the Exterminator cyborgs on this timeline (which appeared to be distinct from the timeline where "Exterminator One" took place), was shown in a single story entitled "Steele" that appeared in 1994 #19, which featured the story behind the first Exterminator cyborg on that timeline (Corben Steele, as opposed to Peter Orwell of the "Exterminator One" series) having a major crossover with Demian Hunter early in the latter's career, as well as Karas Hunter when he was just a young child.
That story is discussed in detail in the last entry for my index to the Exterminator One series.
Lastly in regards to the Hunter Timeline, Demian Hunter had a crossover during the last two stories of his series with a lethal girl who was of the Bathory clan, obviously descended from Countess Elizabeth Bathory, the infamous real life wealthy noblewoman and serial killer of the late 16th/early 17th century who has been incorporated into much vampire-oriented fiction occurring in the Wold Newton Universe, including as an ally of the soul-clone Dracula-Mathias of the Castlevania video game series, and as an apparent consort of yet another soul-clone, Dracula-Lykos, in one of Don Glut's Dracula movies of the previous decade (starting with The Erotic Rites of Countess Dracula).
Note: I have thus far found no conclusive evidence that Maximilian Myron Mac Tavish's alternate future timeline is the Hunter Timeline, but his brief appearance alongside so many other Warrenverse luminaries in EERIE #130 makes it clear that his future time track nevertheless diverged from the Warrenverse proper, much as the Hunter Timeline did, and is thus accessible from it even if it is a distinct time track.
The two Warrenverse series "Curse of the Werewolf" and "The Mummy Walks" not only had a major crossover with each other after the end of the former series and towards the end of the latter, but they actually merged into one series for two important stories that effectively concluded both of their storylines. The full skinny on that can be seen from reading the later entries of both series in my indexes to Curse of the Werewolf (specifically, "To Cure This Curse" and "...There Was A Were-Mummy"), and The Mummy Walks (specifically, "A Battle of Bandaged Beasts" and "Death Be Proud").
The silver Hell Bands and the titular demonic beings they contained from Warren's "The Demon" series had a major crossover with Vampirella during the later days of her series under Warren Publishing--which included a notable cameo from Jedediah Pan of "The Demons" series, who created the Hell Bands--and this crossover is discussed in detail in the final two entries of my index to The Demons series (which covers the Vampi stories from Warren's VAMPIRELLA #92-93).
Warren's classic "Night of the Jackass" series was briefly revived by Harris Comics as a one-shot comic called HYDE-25, and its main character, Sam Bishop (a.k.a., the Jackass), along with many elements related to the Hyde-25 serum, directly crossed over with Vampirella during her Harris days in VENGEANCE OF VAMPIRELLA #9-10, and this storyline had major ramifications for Vampi's long-time supporting character and lover, Adam Van Helsing. The full skinny on that can be found in my index to the Night of the Jackass series.
Bishop Dane, the extratemporal Old West ancestor and regular ally/supporting character of the Rook, had a major crossover with Dax the Warrior (who previously had his own long-running series by Warren) in the stand alone story, "The Warrior and the Gunfighter" from EERIE #120 (the Rook also appears briefly in that story, but he doesn't interact with Dax; so does Quarb, who interacts directly with both Bishop Dane and Dax), the details of which can be found in the last entry of my index to Dax the Warrior.
I do not yet have indexes for "The Goblin" series (though one is in the planning stages), nor an index for the big book-length crossover tale from VAMPIRELLA #50 (ditto), but the Warren hero called the Goblin first appeared in a solo story in EERIE, while more modern successors to the mantle have crossed over with the Rook before the completely modern (i.e., early 1980s) version gravitated to his own series in THE ROOK (where he had a major crossover with Restin Dane himself), and then into three issues of his own mag before Warren declared bankruptcy early in 1983.
In VAMPIRELLA #50, there was a guest appearance by the sorceress Fleur (who had her own series in Warren's VAMPIRELLA) and a briefer original one by Will Eisner's classic crimefighter, the Spirit (whose stories Warren was reprinting at the time, first in EERIE and then in the Spirit's own mag for a few years). At this writing, I do not yet have an index for the "Fleur" series completed, but I will undoubtedly create one in the future.
Lastly, there was a strongly suggested crossover between Peter Hypnos in the latter's Warren series "Tales of Peter Hypnos" with Lovecraftian elements, including the apparent journeys to the Dreamlands regularly made by the series' titular character--the Dreamlands being described in various stories by Lovecraft featuring his character Randolph Carter--and especially the last story in the series, "The Silver Key," which greatly alludes to the story of the same name penned by Lovecraft, both of which describe literal silver keys with the mystical ability to open portals that lead into and out of the Dreamlands. The whole skinny on Peter Hypnos can be found in my index to Tales of Peter Hypnos.
As a relatively minor but possibly significant crossover, it's my personal conjecture that the Gambino crime family (possibly inspired by the real life mob family of that name) who appeared in the second story in the "Exterminator One" series was likely the same Gambino crime family who had such a prominent role in The Butcher series (which took place eight decades earlier; click the hyperlink to check out my index to that series).
There were other crossovers of lesser note in the Warren chronicles, but the above should be enough to digest for now lol :-)