The Recipes post above makes me think we really need some kind of almanac for alchemy in-game, at the very least the recipes we already discovered should show a greyed out potion before we mix it, because let me tell you, I experimented just a bit with it, and I could swear to you that the recipes are semi-random. Probably because flowers move around in the right-hand menu after mixing.
Some actions shouldn't be restricted behind relations with the Prisoners. And I'm not even talking about the obvious here, clothing is kinda hard to get by if you're hunting for particular sets and it's a wee bit silly that I need to befriend my captive just before stripping 'em naked and selling them off to the market. But yeah, the obvious too, I guess.
On the topic of clothing, maybe a specific room to allow crafting/buying it from the market? Heck, you wanna be fancy, make a whole unlock tree for the market in the research tab, for improving the rarity of the items offered.
Additional button for invasions, 'Charge the Invader', maybe? Now that we have fancy new Keephers origins and a warrior among them, it would be neat to have your boss in the throne room hidden safely behind wave of traps and minions, but able to go solo some schmuck if they feel like it. Would help with their levels, too.
Maybe have all the unlocked races appear at the altar to be 'buyable' with gems, but instead of making it a summon, let it be entice. So that there's an event that they arrive under Altar's spell and much like already existing ones, you can either send them away or try to recruit, resulting in 50/50 of whether they agree or shake the spell off and attack/flee.
Overall, I'm quite impressed by the progress made in 0.15, though granted, it's been a long while since I last played DKH (I believe it was even before art change). It was more of a toy for a short affair back then, but now it's, imo, a playable game with a bit of jank here and there.