Trying to clarify how this works, originally I just said "you may play an additional land during each player's turn", but it isn't specific that you can play a land during an opponent's turn. But flash technically is a term for spells, so I'm not sure on this wording. But the idea is a bit like a Prophet of Kruphix just for lands.

The timing ia iffy (e. g. Does the "empty stack" rule still apply?) and the ability is directly contradicted by the current rules anyway.

How about "Once during each opponent's turn you may put a land card from your hand onto the battlefield any time you could cast an instant."? It would be even easier on complexity as an upkeep trigger.

Sure, there are "spell lands" that you want to push out with certain timing in mind, but the pure ramp this already provides is potentially insane.

Altered text to your suggestion. Still potentially too OP at 2 mana? It's definitely more powerful than Asusa, Lost but Seeking, so maybe I should increase it to .

It's a better Burgeoning plus part of a Seedborn Muse. Burgeoning is powerful, but you run out of lands to play pretty fast. The untap side is super powerful, especially in blue. I'd start this at 4 mana

I put the cost up to to be safe for now. Almost makes me wonder if this should just be an enchantment at that point.

As an enchantment it would need to be even more expensive, because creatures are easy to kill

That is true. Legendary creature it is then.