jdsgn: Are you sure about this? Yes, WotC owns the rights to the Planescape setting, but not to the game PS:T. Interplay had a license for 2 D&D settings (Planescape and Forgotten Realms) so I guess the rights to the game (to the storyline and characters) and the source code belongs to them. BioWare owns the Infinity Engine used in the game.
Oh sorry, I read too fast and thought you were referring to the setting itself, not the game. Although Wizards does own the intellectual property, which makes things messy. Interplay (the original "owner" of the game) went out of business then came back (a couple times), the licenses for distributing their games got bought out by a couple other companies, some of whom also went out of business or sold some licenses to other parties. And there are different publishers for the U.S. and Europe, which just confuses things even more.
So, Wizards could definitely have someone make another game in the Planescape setting. The real question is whether their license allowed Interplay to retain IP rights to the character or if Wizards retained all IP rights. If its the latter, Wizards could have anyone create a sequel with the same characters. If its the former, then Wizards probably doesn't want to jump through all the legal hassles to try to figure out where the rights to the characters has landed.