When you realize that Rick and Morty met the simpsons, who had a crossover with futurama(Who met Finn and jake from adventure time, and the Doctor can be seen in the background a few times) , and family guy (who crossed over with American dad and some Star wars AU stuff) and Doctor Who is connected to the hitchhicker’s guide via in universe refrences and in Phineas and Ferb They’ve crossed over with Star Wars (AU) and Marvel (who have made Doctor who comics in the past). And there’s the AU Star Trek/Doctor who crossover. and the TARDIS was seen on Red dwarf.
To add more, DC and Marvel have had multiple crossovers.
And if we look at the roster for Marvel V. Capcom (which is connected by Marvel’s AU’s) Megaman is a contestant, who has had crossovers with Sonic in their comics.
Also Sonic and Megaman are both contestants in Super Smash Bros.
In ssbb Solid Snake makes an appearance and in Playstation Allstar Battle Royal Raiden makes an appearance (Snake and Raiden share a universe) Which brings in PABR.
And if we wrap back to DC, In Injustice: God’s Among Us, Scorpion makes an appearance as a playable character which brings in the Mortal Kombat Roster (which brings in Freddy Krueger, Jason Voorhees, and the Predator)
The Predator also brings in the Alien Series because of their crossover.