I didnt want this to get abused by fastbond/exploration with lands that produce multiple mana (ie, those new CIPT bounce lands). When I think about it though, that would be a 3-card combo, which isn't all that great.
How about:
XXNoNameXX
If a land would come into play tapped, instead it comes into play untapped.

: You may play an additional land this turn.

: Add one colorless mana to your mana pool.
It's a 2 card combo if this can produce mana the turn it comes into play. You get a first turn

(ok, not that great), second turn add

to your pool, play a Karoo (untapped), tap it for

(for example), pay the effect allowing you to play an extra land this turn with the bounce ability on the stack, bounce this and replay it. You then get a second

from it. Untap turn 3, add

to your pool, play a second Karoo, bouncing this and have

available to you.
Probably a very good two card interaction. I'd put the ability cost up to

if it was a current standard card but otherwise it's pretty sweet for the older formats.