I mean, it's a Creature-Sponge...

Is this intended to be a pun?

Nice cycle.

Interesting. I don't think it's possible for lands to have mana costs, though... I mean, I guess they could? It hasn't been done in 25 years though, even on Dryad Arbor, which to me seems like it should have a cost, but doesn't.

I don't know the rules well enough to say it prevents that, though. It's just never been done (for a reason, it seems)

Lands aren't spells though, so they can't be cast. Playing a land isn't casting a spell. Sunburst has no effect, even if you paid mana to play a land (which you can't. You can add mana for any reason, but you can't just spend it on nothing).

This set is an interesting concept, but I predict it is next to impossible for anyone to develop a set like this. The artifact/enchantment creatures isn't awful, but there's only ever been one land creatures... I've made some too, but I wonder how much room there is for design, and how balanced they can be?

Nevertheless, you might as well make a new mechanic or just add some text, because this is far different from how sunburst works.

Maybe you could do: "When ~ ETBs, you may pay any amount of mana. For each color of mana paid this way, put a +1/+1 counter on this creature". Or you could keyword that, if you plan to use it ofte.

This however does work with putting lands on the field. If you don't intend that, maybe you could write: "When you play this card [from your hand?], you may pay any amount of mana [...etc]"

Also, if you don't pay mana for this, or don't get the option to, it dies as soon as state-base actions are checked (0 toughness creature in play == graveyard time)

Paying a land's mana cost is optional.

Lands with mana costs will count toward devotion.

Sunburst for land means you can pay any mana as you play the land. (It won't work if you put it on the battlefield in any other way.)

Terraformer is a creature type for lands that transform into creatures.