I plan on running a Planescape campaign in 4e. Of course to do this I'll likely have to make a lot of new races, feats, powers modify existing material, ect. So from what is known of 4e right now what do you think could/will have to be done to have a 4e Planescape campaign?
Tieflings will be in the PHB, but may have to be modified. I think it would be cool to have a variety of racial feats for the Planetouched races that emphasize they're specific heritage would be cool, so a Succubus descended Tiefling would be different from a Tiefling with a Glazebru for an ancestor, which would different than one with a Barbazu for an ancestor as so forth. The lack of level adjustment and the fact that most racial abilities will come from feats will help 4e Planescape games a lot as races that weren't playable at 1st level in 3e will now be.
It's really too early to say. It'll be a lot of work, that's for sure.