My Take on a Dual Land

[SmashPortal]

Vermont
Legendary Land -- Mountain Forest
({T}: Add {R} or {G})
Whenever Vermont is tapped for mana, skip your next combat phase.
"It's so pretty here."

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Edit 1: Added mana hint text as per BOVINE's request.

[idSurge]

Would be nice for decks that dont care about combat in the least.

[Mookie]

Huh, that's a pretty interesting downside. Two notes I would add:
-seems very powerful due to how many decks don't care about combat (many control and combo decks, for example).
-seems like it could cause memory issues, especially if you tap it during your second main phase, or if you're untapping it multiple times.

Interesting to contemplate lands that have skipping other phases as downsides - draw step or untap seem brutal, skipping your upkeep can be a downside. Skipping main phases seems like it could pretty balanced - possible to function without it, but necessary for almost every deck. Hmmm....

[user_938036]

It's a downside that is easily mitigated and has significant memory issues if you have multiple which you will want considering it essentially has no downside. These are nearly unplayable with their downside and original duels if you can ignore the downside which is fairly easy.

[BOVINE]

I'd add (T: Add r or g.)

[spacemonaut]

Memory issues would be managed like this:

Whenever you tap ~ for mana, put a serenity counter on it.
If you would begin your combat phase while ~ has a serenity counter on it, instead remove a serenity counter from it and skip that combat phase.

If you have one of these that you've tapped for mana several times in one turn then you're able to keep track of your combat phase debt by the number of counters on it and tick that count down one combat phase at a time (or add to it).

This works fine with multiple lands of this style too. (Maybe you have Mirror Gallery or you're running more than one in the cycle.) Each time a combat phase might start you pick one of their replacement effects to apply and remove a counter from just that one. You won't remove a counter from all of them at once because once the first one's replaced the combat phase, there's no more combat phase left for the others to see and replace.

But, yeah, for decks that don't care about the combat step, this is effectively a legendary Taiga|ME4. For control decks that take a great many turns to eventually win at the combat step (e.g. with a solitary Aetherling) they'll just wait a few turns to let their combat phase debt end.

[Megatog201]

Or you could remove the memory clause entirely by putting something like
Tap add G or R. Skip your combat phase this turn. This ability cannot be activated if you attacked with a creature this turn.
Not functionally identical, but close and removes all the unnecessary elements.