I'm pretty convinced it's a dead end. Yes, there is some space in between treasure cards and non-terminal actions, but it's very narrow. And I don't think it would create interesting strategies, in fact, I think it would simplify the game. The point of action cards is that, if you're not playing an engine, but you use a drawer, you don't want too many of them. A BM + Council Room strategy f.e. doesn't like to have markets in it, since they won't be worth anything when drawn by CR. Now, if you use non-action cards, that prospect is nullified. A non-action "+2 card" vanilla card is way easier to figure out than a lab, because you can just equalize its worth with twice the average value of any card from your deck. And if your non-action doesn't draw, odds are it would prefer just being a treasure.