Definitely interesting. Too much of a drawback for a enter-tapped dual, as they still get enter-tapped plus an advantage, but could be on an enter-tapped any-colour land or an enter-untapped dual. I'm not sure how how big the drawback is: it's potentially quite a bit, but most of the time your opponent won't get the mana at a time useful to them.

That was my thinking - it's a huge disadvantage if the opponent can use it; but I didn't think they'd be able to use it often enough to make it enter-untapped safe. Any colour (or any three?) would maybe work better - especially since the opponent is more likely to get a use out of the mana that way.

Or perhaps the "Each player adds a mana of a colour a land they control could produce" ability, so everyone gets hopefully-useful mana, but it doesn't do fixing for the opponent.

Very interesting. Much more printable now that mana burn doesn't exist :P I think this simplest power level is interesting enough balance that I'd try it at this first.

Hmm, in fact, I'm wanting a mechanic for the rare dual cycle in the second set of the Clockwork Wings block. Might I consider stealing this one?

Sure, give it a throw :)