Since some people have said they never successfully reconnected, I'll chime in and say that I have, and I've also had opponents do so. But there's the five-minute timeout thing to contend with; you need to get your connection back fairly fast. I think what jsh meant by "reconnect" and "choose whether to make a disconnected opponent to resign" isn't about the five-minute timeout, but saving a game state and being able to pick it up at any future time. You'd need an interface like "my games in progress" that highlights whether the opponents for those games are online and available, i.e. not in the middle of another game.
So if someone disconnects and I think they'll be back, I can just say "pause this game" instead of forcing a loss on them. Later on that opponent and I can resume our game, whenever we happen to both come back online. Or better yet, just let all players agree to pause a game any time. If a player is disconnected and the other(s) say "pause this game," assume the disconnected player consents.
If you're running out of stuff to do, you might also want to let users use the bot AIs for simulation.
Not sure what you mean by this, but I'm gonna jump off the deep end with it: publish an API for us to write our own bots. (Note: I realize that would almost definitely allow us to make alternate user interfaces as well as bots, but that's not necessarily a bad thing.)
Repeats of other people's things:
Ability to choose who goes first in unrated games (useful for league/tournament sets among other things).
Option for identical starting hands (if both players agree to it).
Rematch button when a game ends, offers to play again with the same opponent(s) and rulesets. If they accept, skip matchmaking/lobby and go straight to the next game.
The controversial one:
Option for some small amount of kingdom control in pro games, whether it's iso-style veto or Donald's hate-list idea or something else along those lines (there are multiple long threads debating the best way to do this, if it should be done at all, on these forums).
Saving the most important for last: Don't lose features you have now! That includes ones provided by Salvager, since it's pretty ubiquitous. Seriously, I'm willing to wait longer for you to implement everything Salvager currently does before releasing the new interface.