Slow enough--e.g. correspondence time controls--I would guess that the top GMs are still favorites, although I doubt we'll ever see such a match. I imagine some lessor GMs have given this a try...

I favor Kasparov's definition: as long as humans win any games, it's enough.

I'm not sure people are trying to solve that problem. E.g., no advantage from an engine-engine perspective to make the computer stronger at openings or (late) endgames when you can just look those positions up.

maybe a combination of a normal chess board normal movement of the pieces and general chess rules aply but with a sortof magic the gathering vibe. Pre match you construct your 'add ons' from a wide variety of choices (these stay hidden for your opponent untill you play them) so you can assign special powers to your pieces during the match and most importantly combine them during attacks.

stuff like, knight becomes statue after a move and cannot move for the consecutive turn but is also invunerable to attacks. Or turn an adjacent empty square into quicksand. so noone can stand on it for some period. Obviously needs balancing and exceptions and determining how many add ons are allowed. Maybe exempt the king from such trickery so you still hunt for normal mates.

I think humans will be better in such a game when you have to use your imagination much more instead of cold calcs. Moving your pieces and combining unknown add-ons. Dont think computers will have the 'feel' for it.

I think its sound like fun and If someone good designs it I think you have a game thats very much like chess with something extra.

Well, the Arimaa game I mentioned realizes this kind of attempt. And indeed, humans still have a big edge.
http://en.wikipedia.org/wiki/Arimaa

OK, you can play it with a chess set but thats about it. It has no resemblance with chess anymore. I was thinking of something to make the game bigger (more moves possible) without making the board larger and came up with the add-on idea.