JB's already done his "other" dream crossover for me- Darkseid vs. Galactus.  And it did not disappoint!

Otherwise, JLA/Invaders sounds good to me, as does Human Torch/Captain Marvel (my two favourite Golden Age heroes).

Here's one that won't interest anyone but me, probably: 

I'd love to see a DAREDEVIL/NIGHTWING crossover by Mr. Byrne.

BATMAN & CAPTAIN AMERICA is one of the best stories by JB ever. It's always a pleasure to see a modern super heroes comic book settled in the WW II days.

A Superman/Spider-Man team up wouldn't be bad, I have thought it since CHAPTER ONE.

From my end of the equation, I'd much rather have the challenge of finding stories that worked for disparate characters. One of the methods I considered for "plotting" an ongoing GENERATIONS series, for instance, would be to have two shoe-boxes filled with slips of paper on which were written the names of the characters (same names in both boxes), and pulling one or two from each, at random, to tell me who I would be using in the next issue. Really unlikely combinations would be, I thought, the most fun to play with.

I wonder if this is in part the competition factor, people want Similar characters together to show once and for all, which of the two is better.  Certainly the "Vs." threads and chat topics have been hale and hearty for years over this kind of thinking/desire - which if someone thinks about it, it's unlikely either company would agree to let THEIR character be shown as 2nd best (without some loophole or balance.)

Sometimes there could be something interesting in taking characters thought to be similar and using a pairing to show up the differences, i.e.. Thanos and Darksied who really aren't too much alike at all, tho' one was certainly based on the other, could stand a good juxtaposition.  But otherwise, I think the story has to be there to be told, and a simple "let 'em slug it out to see who's standing at the end" would be very boring quite quickly.