I can see Stark and Wayne joining forces once they figure out they're on the same side. Iron Bat armor would be inevitable.
Would Dr. Strange wear the Helmet of Nabu?
Would Quicksilver join the extended family of Flashes and other speedsters, or is he too much of a jerk?
What would happen if you exposed the Atom's dwarf star matter size-changing suit to Pym particles?
Romances: Iceman and Ice? The Human Torch and Fire? The other way around would be tricky...