an idea i was playing around with, not sure if its any good or not.

You don't need the 'face up under this card' for the ability to work.

looks cleaner that way, thanks

Why not ": Add , , or ."?

because its been done before? Plus it was just an idea that i didn't want to forget

So the Forest has to be untapped, but the Plains doesn't?

whatever land you exile has to be untapped

It needs to say "untapped Forest or untapped Plains" then to avoid ambiguity

So... is that assymetry intentional where this always produces at least one green mana? I'm not on the up and up with the set's themes. Is that playing into it?

This is not a set, just a dump for random ideas that may make their way into a set in the future or may not.

I can see it working; kinda wants a different graident shading to reflect it's mostly green and only a bit white.

Has worse memory problems than it looks like though; usually you would exile the plains underneath it to remind you - but land gets stacked like that anyway, so you could easily accidentally make from this - by forgetting that the plains is exiled.

Not even 'forgetting'.

On the downside - it's pretty specialised, isn't it? If you've got a way to untap a small number of lands, it's useful. Otherwise, just plaing a forest instead seems to be just as good?

yes its specialised, was just playing around with an idea though, not a 'finished' card by any standard.