It's the 50th anniversary, so wouldn't it make sense to release a bunch of products that celebrate that history? Some new glossy coffee table book on the history of D&D, a new printing of the white box, an update of the silver anniversary collection to a gold collection....
I know what I'd do, but there's no way WotC would.
Given that it has to be "Backwards-Compatible " with 5e. So either a 5.5 or a 5e.
1. Make a big deal of it being a 50th Anniversary version. (Okay, this part, they will do, with alt covers and all that.)
2. Let the Marketing team go ape shit with random swag. Keychains, backpacks, t-shirts, dice, all that. (This, they will do.)
3. Pull the Dragonborn, Teiflings, Warlocks and casting subclasses of Martial archetypes out of PHB. Add the Cavalier and Scout subclasses. MM and DMG are fine as-is. Add in whatever monsters and items that have come out since 2014. Big "50th Anniversary Edition!" on the front, even if you change absolutely nothing inside.
4. Put out a 256 page Forgotten Realms setting guide with pull-out map. It's your official setting. Say it's damn name.
Put out a Dragonlance book, Ravenloft setting book, Spelljammer, Dark Sun, and Planescape books. All under a "D&D Classics Revisited" tag line.
5. Put out all the MtG settings books under a "D&D Expanded Worlds" tag line.
6. Throw a version of the 1e Wilderness Survival Guide and Dungeoneer's Survival Guide together in one book, along with some map making advice, yada yada. Throw the Artificer, Warlock, additional races and subclasses into some sort of PHB 2. Call them "D&D Expanded Options" or whatever.
7. Profit!
8. (Make massive staffing changes, but they won't make the ones I would.)