How about:
Illusionist$4 - Action-Reserve
+$2.
Put this on your Tavern mat.
When you play an Action, you may call this, to name a card and a card type. All of your copies of that card have the chosen type until the Action is resolved.
Note that I am following the official comma usage.
I think this simplifies it a lot. It narrows the window of use to the Action phase and doesn't have any extra complications like having to forbid Curse type. Is there a time when that matters now? With the existing cards, I don't think so.
This wording does make certain combos weaker. If you want to bring it back up, you could instead have the condition be "Any time while you are resolving an Action [you played]". The "you played" is extraneous but can help limit potential confusion (e.g. "does it count as me resolving an Action when you play Noble Brigand and I have to reveal cards from my deck?").
Another issue to consider is that this Reserve card can actually be dead on many boards. It's not really worse than Conspirator in that regard, but still.
Also, since it is now so limited, I think $4 is probably too expensive.
@GreyICE, if a Treasure allows me to play e.g. Village during my Buy phase, can I then play more action cards? Village gave me +actions, after all. The answer is no, but many people would find that unintuitive and confusing.