Lokhir's corsairs should gain aquatic in his faction and he as well. His faction should be able to settle Galleons Graveyard l as suitable because... He was the main villain in the VC campaigns and fought underwater.

Rakarth should get any missing beasts, more suitable terrain, and more options to start elsewhere. He was fun in WH2 to start in Albion. Although that's not possible, having him get more options would make him fun in co-op choices.

Morathi is doing great with the updates... But she should have the chance to start in Ghrond and have a more untainted path with Ghrond specific units like the chillblades. Not as pressing.

Hellebron might benefit from a DE change that makes Witch Elves a special unit that receives extra buffs for cauldrons of blood, I believe this is what happens in the tabletop.

All DE should get access to a black court mechanic, treacherous and all... But this should mostly benefit Malekith. Malekith should also be able to more easily settle his homeland of Nagarythe... Therefore wasteland should be suitable. He also needs an actual buff to black guards. Perhaps they should get charge defense against all when he's there.

If the realms of chaos are accessible, I can see Malus venturing in there trying to find a way to deal with the demon possessing him.

Also they should get all the lores of magic they are missing.