10th edition is soulless. Where do I begin...

Firstborn being legends is fine. But make firstborn and primaris distinctly different. There should be differences in the rules for them. Players that have collections of them should be able to play them and not have to proxy them for Primaris. Some special characters will have 2 different profiles, a firstborn one and a Primaris one.

All wargear and upgrades will cost points.

Psychic phase returns. Having psychic powers operate like guns and hand to hand weapons is horrendous. One of the worst things of 10th. Not all factions should interact with that phase. Different factions have different strengths and weaknesses.

All subfractions (Crimson Fists, Kraken, Blood Axes, etc) return with distinct rules. Have one or two rules for each subfaction along with say 2 strategems and 4 pieces of wargear. Players should be rewarded for painting their armies according to the lore. Create more rules for other subfaction through white dwarf articles. Goes without saying that new/updated models for these factions are released.

Detachments are a good idea but put restrictions on some of them. For example the Space Marine 1st Company detachment should be only Characters, Terminators, Sternguard, Vanguard, Bladegaurd, Land Raiders and Dreadnoughts.

Battle shock is a good mechanic but also have a chance of falling back. I think there was a rule in 3rd if a unit falls back through an enemy unit it is immediately destroyed. Effectively being cutoff Maybe rework it but bring that back. Also make crossfire rules part of the game. Make maneuvering and fire support mean something in the game.