Vertical slabs and stairs are never gonna happen as it currently stands. There's no point in asking for them.
Skeletons and their variants SHOULD be able to use crossbows, yes. I think pillagers should also be able to use bows and melee as well. Would be great for custom maps/mobs.
RIP iceologer indeed :(
baby parrots would be nice, same with horizontal glass panes.
Roses are still in the game, just as bushes. It doesn't really makes sense to just have a single rose, cause they don't grow like that irl
CAVE UPDATE'S COMIN' BABIEEE!
As for the advancement, that can easily be done with a datapack, like pretty much any other advancement anybody has ever wanted.