My bet is that 6e will be 5e's engine, with some tweaks and new artwork. The biggest revisions will be in the PHB and be what is included with each class as subclasses, the list of spells at the back of the book, and possibly some revisions of the feats.
I suspect that going forward the era of experimenting with D&D's game engine is done - the rules are going to be as locked in place as Monopoly or Risk are - those games have subtle changes every few decades but mostly are recognizable as the same games you could buy in the 1970s with modern artwork (or in the case of Monopoly, completely unchanged). The RPG market is a lot more mature now and D&D "knows" what kind of game it wants to be - it doesn't have to chase new ideas and new mechanics anymore.
In fact, instead of a new edition I expect them to release just a revised Player's Handbook eventually and leave everything else in place. No "6th edition" push to get everyone to upgrade at all - just get folks to all buy a new PHB while leaving everything else alone.
(Also I fully expect them to continue to play with the boxed sets in stores. Eventually if the rules stay stable enough they might embrace for D&D starter sets what they've done for licensed properties on their board game side and have one-off box sets around licenses based on the D&D engine where they can. I don't think they're quite there yet - and there's the argument to be made that you can't recycle enough of a Starter Set to make it worthwhile like you can with a Monopoly game so maybe it wouldn't work - but I wouldn't be surprised.)
I suspect that going forward the era of experimenting with D&D's game engine is done - the rules are going to be as locked in place as Monopoly or Risk are - those games have subtle changes every few decades but mostly are recognizable as the same games you could buy in the 1970s with modern artwork (or in the case of Monopoly, completely unchanged). The RPG market is a lot more mature now and D&D "knows" what kind of game it wants to be - it doesn't have to chase new ideas and new mechanics anymore.
In fact, instead of a new edition I expect them to release just a revised Player's Handbook eventually and leave everything else in place. No "6th edition" push to get everyone to upgrade at all - just get folks to all buy a new PHB while leaving everything else alone.
(Also I fully expect them to continue to play with the boxed sets in stores. Eventually if the rules stay stable enough they might embrace for D&D starter sets what they've done for licensed properties on their board game side and have one-off box sets around licenses based on the D&D engine where they can. I don't think they're quite there yet - and there's the argument to be made that you can't recycle enough of a Starter Set to make it worthwhile like you can with a Monopoly game so maybe it wouldn't work - but I wouldn't be surprised.)