In all honesty, I'll be disappointed if all Nintendo does is allow us to play old ROMs on the Rev. Though that's okay, but that should be just the beginning.
Now, if I can play classic games in multiplayer over the Internet (such as Toe Jam & Earl with a friend in another state), that would be great. If they add new levels to classic games (i.e., new levels for the original SMB or new worlds for classic Zelda games), that's good too. If they allow us to play old prototypes that were never released, that's very cool.
And even cooler would be for Nintendo to release NES and SNES development kits that allow anyone to create new ROMs and submit them to Nintendo for possible distribution over their network. Nintendo could have contests with prizes for the winners and such, like Lik-Sang and others have done in the past. It would be a way to continually add new classic gaming content to their system without devoting any of Nintendo's own development resources to it.
If Nintendo approaches the Rev's retro content with new ideas and ingeniuity, it could be the greatest game system ever created. If they just say, "here's just a bunch of old games you can play" and leave it at that, then it'll be a major disappointment and one of the saddest lost opportunties in gaming history.
Hopefully, Nintendo has the vision to do this right.