If Runebound 3e would be just a upgraded 2e it would be boring and dissapointing from my perspective. The market changed and there's a lot of other great adventure games even in FFG library.
So it all depends on how FFG would make such a game look like,
I agree that making RB3e a slightly modified version of RB2e would be disappointing. Fortunately that's not FFG's style, generally speaking. When they release a new edition of something, they usually have more than just superficial changes to make. That said, I've noticed FFG has started popping out "revised editions" of a few games which are exactly that, so here's hoping they don't try for RB2e "revised edition."
Where we part ways, however, is in the idea that RB2e was not a good adventure game. I liked (and still like) RB2e just the way it is. I think it's got plenty of theme and really gives players the freedom to do what they want. That's important to me in an adventure game - the idea that I'm going out and exploring this world rather than following one of a limited series of paths around a board.
The only real complaint I've heard about RB2e (that strikes me as a legitimate problem and not just personal tastes of the complainer) is downtime between player turns. Even that doesn't bother me so much, personally. I suspect that FFG recognizes that, too, which is probably why they're so slow in bringing out RB3e. Sales of RB2e were no longer sufficient to keep it in print, but they haven't worked up enough valid changes that legitimately improve game play in order to make RB3e official.
As for your suggestions, it seems like RB2e already had most of those things in spades:
A board game with a good card mechanic maybe similar to the WFRP 3e with more then one variant of play would be a very interesting.
I'm not familiar with WFRP 3e, but RB2e had a good card mechanic in the challenge card system, IMHO. There were plenty of adventure variant expansions released which changed game play in significant ways, and usually each box expansion added or changed at least a few mechanics as well.
Custom dice would also make it in plus but they should not be the main mechanic.
I don't necessarily get anything special out of a game having custom dice, myself, but RB2e did have the movement dice which were customized, but not part of the main mechanic.
Adding variant to play solo/co-op or player vs player would make it even better.
PvP was admittedly a failing for RB2e. It wasn't done well. Solo, on the other hand, was practically inherent in the game engine. True co-op was also mostly absent, although it would be easy enough to play Mists of Zanaga as a co-op by saying everyone needs to defeat all the primal gods together in order to win as a team.