There isn’t a lot of stuff I think should be removed, but a lot of things need to be restructured. And mono-unit hardcover “Codexes” are the first thing that needs to go.
That sums up how I feel as well. A long time ago, in a galaxy far far away ...
There was a game called "Warhammer Fantasy", and during one of the various editions for that game they released a series of books that were intended to be filled with units that could complement existing armies. I think one of them might have been called "Dogs of War" but I never followed Fantasy all that deeply so I could be totally wrong. It was sort of like
GW saying "Here's a bunch of units and really small factions that don't have enough substance to be books on their own but could be used to compliment several existing factions."
I'd like to see something more along those lines. So maybe make "Xenos", "Imperial", and "Chaos" books and have some of the smaller factions rolled into those. For example, you could have Imperial Knights, the Inquisition and the Assassins all in one Imperial allies supplement. This would also give them a great way to expand on things like Tau auxiliary troops and Eldar Corsairs without having to produce entire books and a ton of fluff for each. Just a few extra units, a page of stats/ rules for each and maybe a paragraph or two of basic fluff is all you would really need.
EDIT:
I'm just wondering. Honestly i'd get rid of Slaanesh. He just seems real boring fluff wise in my opinion.
The only thing is, if you dump Slaanesh, then you have to rewrite like 90% of the Eldar fluff (which would inevitably lead to ripple effects in the other factions as well).
IMO you have that issue with most of the major factions.