You have a base of Huel. Dropdowns to mix in
Protein (Isolate, pea, hemp, etc)
Creatine (Creapure, Mono, Hydrochloride)
other premium ingredients
Flavouring system (chocolate, bananas, etc)
It’s what I do anyway - one part Huel, one part protein shake. I’d be all over a create your own system and willing to pay for this service.
The trouble is, although the daily cost can be quite low, all the recipies have a fairly large initial outlay for ingredients that might blend into something deeply unpleasant and not necessarily healthy (the iron levels always seem to be through the roof).