Ooh, I like the idea of bringing back the Niv-Mizzet, the Firemind ability. Adding a mana cost does let you reduce the rarity, certainly. Wizards would probably disagree on this being common these days, but I still think Prodigal Pyromancer and Samite Healer don't cause any problems at common.

But, um, shouldn't it have flying?

I've actually been thinking about this problem. I know why adding an mana cost to Enlighten is be a good idea... to stop it from going berserk. It also seems to me to be a bad idea. If Enlighten only appeared on a couple of cards, then it would be fine. But having to continuously make the decision of whether you want to pay mana to activate Enlighten, or just cast your spell for the turn could be frustrating when drawn out between 15 separate cards, 5 of which are common. I think we might have to go with the broken version if we expect people to like it...

Oops. I entered a bunch of these really quickly last night, and missed flying on this guy.
Like everything, Enlighten probably needs to mix of having a mana cost and not. A few cards can be free, but some should cost. Then it will be like mechanics in other sets: some cards with them aren't that great, others are good.

Tom got really excited when he used this card with Manacycling as a combat trick. Pay 3, draw a card, use the to activate the ablity. Manacylcing and Enlighten are awesome together. As a matter of fact, in the future, if we are asking ourselves "What is the proper activation cost for this Enlighten ability?" I would say that the proper cost is probably .

Yay, manacycling and enlighten were designed to work well together; it's pleasing to hear that they're fun and interact well :)

I wonder whether this could work without the mana payment. At that point it's closer to Prodigal Pyromancer or Honden of Infinite Rage, and should perhaps become 1/1, but that might be fine.

Is it annoying that removing the activation would cut into the specialness of Niv-Mizzet? Maybe. It's probably fine if it does, though.

That said, the fact that Niv-Mizzet exists kind of makes me not want to replicate his ability on any card. Meh. It's such a natural choice, though.

Mm, that's true. It is a very natural ability, but Niv-Mizzet, the Firemind did get there first.

I guess if people still enjoy using this as it is - which you mention Tom did - then it's probably fine to keep it at its current effect, with the mana payment.