Not anytime in the near future, but I would plonk down some money for a cross compatible Alternative PHB with completely different races, classes, feats, equipment and spells that could be played discretely on its own or your DM could pick and choose what to include or you could play an entirely PHB based game and still use the current MM and DMG for support. It is probably the best way to get classes like Shamans, Wardens, Warlords, etc into the game without infringing too much on the niches of the current classes.
I probably see something like this more as an end of edition type product before 6th Edition rolls out in 2024.
Before then just a big old 320 page splatbook of subclasses, new feats and spells would hold me over just fine. No theme. Not a themed Swampscape: The Complete Guide to Characters Who Like Swamps. Not a setting tied product with 40 pages of character options, but basically a PHB2 (just don't call it that though either.)