What I would want to see, and what will be are two different things.
The idea for this was already done with Magic the Gathering. The worlds that they have had are a preview of what they are probably looking for with D&D. The ones that sold poorly probably will not be done again, and even if another company owns the RPG rights, WoTC may still get the ability to do a one off. In that light, high contenders that WotC probably would want would be the Lord of the Rings (yes, some think they would not do this to be nice to a competitor...think again...and also...when was WotC nice in the present day?), Marvel, or other properties that seemed to do well. I'd say Final Fantasy may be a thought. Smaller properties listed that people want may be nice, but if WotC doesn't see it selling enough to make up for the investments (bribes to publish the one off to the owners of that IP...which in some instances can be quite massive) into that one off...it probably won't happen. It also won't happen if they don't see that it would generate sales (so...maybe no TMNT) of a certain quality/amount.
What I would love to see would be things like Thundercats, Masters of the Universe...things like that. But I also know that is never going to happen. The Sales cannot justify the expense.
They are looking for things to boost the D&D brand, not just merely exist.