Assuming WotC release 5e as the three core books rather than all the usual splat books that normally appear I'd like them to spend the first year releasing books aimed at specific previous editions. So we'd get a rules book for D&D Next for 4E with specific rules on using/converting the core 5E system to that edition, another book would be for classic AD&D etc.
This would allow people to use the new system to play their favourite edition and could be a cool new take on the rules plus a refreshing change from Martial Handbook I, Arcane Handbook, etc.
This assumes that people who are deeply committed to edition X will be interested in adapting 5E to play like edition X. Why would you do that? You've got edition X already. If you switch to 5E, it's going to be because 5E offers you something that edition X doesn't--which means that you don't
want it to play just like edition X.
I would be much more interested in an "Unearthed Arcana 5E" book, which presents an array of options for customizing 5E in wild and out-of-the-box ways, and doesn't attempt to group them by edition. Let us, the players and DMs, be the judges of which elements of each edition appeal to us. While they're at it, they could throw in ideas that have never been present in the printed rules of
any D&D edition*. E6 rules! Vitality/wound points! Something that does what skill challenges tried to do, but actually works without the DM having to build a ton of stuff on top of the basic system!
Beyond that, I think they can and will profitably occupy the first couple of years just filling in all the usual gaps. Monster books, setting books, Manual of the Planes, PHB supplements with the classes that didn't make the cut for PHB1, et cetera, et cetera. But I do hope they're more adventurous in terms of providing optional rules and variants.
[SIZE=-2]*As far as I know. Now that I've said this, somebody is going to point out that in the Dragon Magazine Director's Cut, cuneiform edition, all of these things are presented as official rules for D&D 1.5E.[/SIZE]