Thinking about it a little more, you might even want to bump it up to $7. I'm thinking about the different engines you can build and how easy it would be to use this card to get them going. One of the problems with engines, especially before they're fully mature, is that you don't always draw the specific cards you need to kick it off. So imagine these engines:
1. +Actions/+Draw engine: If you draw nothing but Smithies, you make it a Village. If you get nothing but Villages, you make it a Smithy. If you have the similar Village/Torturer engine, you can pull the same trick to more devastating effect.
2. Goons engine: Your card can't become a Goons, but it can certainly become a Village to make those multiple Goons you drew playable.
3. Conspirator engine: If you draw some Conspirators without enough non-terminals to kick off the chain, you can fill in the gap. Similarly, if you draw starter cards without enough Conspirators, you can fill in that gap instead.
4. Minion/X: If you draw a hand without a Minion, it becomes a Minion. If you draw a hand with a Minion, it becomes a Conspirator, a Festival, a Bazaar, a Market, a Worker's Village, or any other non-terminal that would provide you +Buys or +Coins as you're cycling through your deck with the Minions.
5. Cursing attacks. Not an engine, per se, but if, for example, Witch was on the board, you'd rather have your card instead if you could get it soon enough. Because Cursing attacks are already incredibly powerful, despite being balanced by the fact that the Curses do run out eventually and become dead or weak cards afterwards. But your card could be a Witch until the Curses run out, then suddenly be a Fishing Village or a Laboratory or something, without missing a beat. That's huge.
So I'm suspecting that this is going to be a lot more powerful than it first sounded to me. It's a neat idea, so I hope it's not still broken at 7. I'm pretty sure it's usually going to be preferable to Bank, Expand, and maybe Forge. But the nice thing is that it IS still situational. On boards with weaker kingdoms, especially if there are no great 5-cost cards, it's probably not worth going for. However powerful it might be sometimes, it's not always a must-buy, and that's a good thing.