I think you could make Royal decree a throne room and still have it pretty viable most of the time. The fact that it's using up only one card from your hand is pretty neat, and gives you the same net advantage as king's court (one free action play out of nowhere). Pro is you don't have to connect it with your other actions, cons are you have no control over what kind of stuff you get to play.
The free card to the opponent is unfair to the other opponents, so I'd mercilessly take it away, and I'd even consider streamlining away the choice of +Action. I think the card will probably be good enough without it, and it simplifies a lot the chains you might get when hitting an opponent's Decree - you don't have to remember what Decrees are +Actions and what are Throne rooms. (I'm not sure I'd want it to be possible to hit Decrees at all, since it could be swingy as hell - play one card, hit opponent's Decree, now your card is 4 cards. Hit another Decree, now it's 6 cards, and that's just crazy, and each hit makes it more likely that you'll branch out more, possibly ending up playing each of your opponent's Actions twice, just by playing one Action card. Next play, you hit Secret Chamber. Big deal.)
I really love the idea.