The best case is they produce the goods and a game that simultaneously plays like OD&D, AD&D Rules Cyclopedia, 3.x and 4e, for any version of the above.
Now I an not convinced that this is possible. So I think that the realistic best prospect is a game that feels line core AD&D, Rules Cyclopedia, 3.x and 4 depending on the options chosen. I think that even at best there will be vocal dissatisfied players upset that blah from book blahblah is missing, or it does not feel right, etc.
The worst is that the wrong element are listened to and that the edition focuses too much in that direction leading to its rejection by fans of other editions. This is most likely if that edition's feel matched most closely with the biases of the desginers.
Now I do not think that this is very likely but would be a total disaster for WoTC and pretty much anyone that is looking forward to this but is not a fan of Pathfinder. the reason I am pickin out Pathfinder here is that Pathfinder is independantly supported, os its fans are covered so to speak.
The other way it could go badly wrong for WoTC is if they have inaccuretely mapped out the size of the edition populations.
Right now we have a number of populations of different sizes playing different editions of D&D. Some are playing the original games, some games produced by others that fit the feel of editions of D&D (e.g. the retro-clones and Pathfinder).
So a key question for WoTC is how big are these populations and how much corssover occurs. The final game should most please the largest population but not so much as to alienate the others. the other element is what aspect is it that binds these populations to their chosen version and can it be addressed simultaneously with the key attractors of fans of the other editions.
If they can indentify the relevant populations and the key elements that bind these populations to their beloved edition. Address that attraction across all the editions then they will have a success but if they mis-identify the poulations or the essential elements that attract a group to their prefered edition they coould produce a game that is poorly supported.
The downside to WoTC if they get it wrong is that the D&D division gets shut down. The downside to the community is that D&D gets shut down. Pathfinder and the OSR becomes what is left of D&D.
Fans of Pathfinder are in a relatively strong position on this, however, Erik Mona has stated here that he believes that the current edition of D&D is still the primary vehicle of introduction of new blood into tttrpgs. I don't know if he still holds that view or if he believes that Pazio is now strong enough to take up the slack if WoTC D&D dissappears. If he still holds his previous position then a failure of 5e is Ryan Dancy's prophecy of doom fulfilled.
