GIMP 2.99 already handles color spaces better than 2.10. E.g. you already have late binding for CMYK (but not early binding, that's post-3.0 stuff).
There's no telling if layer filters will make it to 3.0, I guess we'll see how much is done at feature freeze time. So I wouldn't be too fast to jump at conclusions, Even if it's missing in 3.0.0, 3.0.2 would have it.
As for HiDPI, the three way switch for canvas zooming suggested by Jehan here — https://gitlab.gnome.org/GNOME/gimp/-/issues/5206 — is something I hope they will reconsider. There should be a better way to handle it.
> Does anybody know what is expected to make it into 3.0?
Everything you see in 2.10 point releases (much was backported from 2.99) plus some extras like GTK3 port, multi-layer selection, complete rewrite of linked layers concept, Lua and JS bindings, and various other things. Because of backporting, separating 3.0-specific changes from the rest is not an easy task.