No limits. Any series is free for you to crossover between either yours(if you have one) or each other, and there is no set limit to how many you can fuse.
halo and mass effect
Bionicle, Transformers, and Hero Factory, for good measure.
Borderlands and Halo would be cool.
Transformers and Bionicle would be better!
oh wait there is no limit to what we can do?
oh alright then
Mass Effect, Halo, Transformers, Bionicle, Marvel, DC, Star Wars
BIONICLE and Marvel. Think of the possibilities!
....(I'll include companies to encompass multiple series)
-All good/memorable cartoons from the 1980s-2000s, possibly 2010s
-Many Popular Mangas/Animes
-Any Series with Kaiju
-Metal Gear series
The Lego Universe (not LU, but the universe that contains all lego themes, which may or may not contain LU) + Halo
I once came across a picture of the mata nui robot kick the living daylight out of some reapers. I thought it was epic so i guess mass effect and bionicle ...
Pokemon + Attack on Titan, only because ship Pikachu and the Colossal Titan...
I've always kinda wanted the villains from Ninjago to launch a full scale invasion of Chima. Also, I love interconnecting all the smaller lego themes into the same canon. I might end up writing that out and making a complete fanon Lego universe with a plausible timeline.
Aside from that though, I've honestly never really wanted crossovers. Everything I like seems pretty solid in their respective universes. Of course it's fun to consider what would happen, but I don't usually want anything to get convoluted by converging plots.
Kamen Rider and Ultraman
Dragon Age\Inheritance Cycle, to parody the latter.
Legend of Spyro\Dragon Age.
Legend of Spyro\Wings of Fire.
Wings of Fire\Dragon Age.
I could go on.
I'm personnally not a fan of crossovers,but Halo+Mass Effect i guess would be pretty great
Sherlock and Arrow.
Don't question it, just roll with it.
Jojo and Persona.
Gotta get that Dio S Link
Metal Gear/ Red vs Blue, it would be really cool to see a highly skilled special ops agent like Snake face one of the agents in Project Freelancer, or a sword to sword dual between Raiden and Tucker.
Bionicle + Game of Thrones is what my dreams are made of
Every Megaman series ever. (Classic, Battle Network, Star Force, X, Zero, ZX, and Legends. Marvel v Capcom and Smash Bros. stuff don't count!)
Bionicle + Real life.