Let's say WotC gave us a hypothetical Legendary Tribal Land - Sliver in a future Modern Horizons set, I have it in my hand, and a Quick Sliver on the battlefield, when could I play that land and what rules baggage would I need to know concerning it?
Quick Sliver now says: "Any player may cast Sliver spells as though they had flash" (C.R. 108.1). Thus, even if there were a Sliver tribal land card in the game, your question would still be moot.
In any case, even if an effect could allow you to play a land "as though it had flash" (that is, any time you could cast an instant [C.R. 702.8a]), that wouldn't change the rule that you can't play a land, "for any reason"—
if you have already played as many lands as you're allowed to this turn (usually one) (C.R. 305.2, 305.2b), or
In any case, even if an effect could allow you to play a land "as though it had flash" (that is, any time you could cast an instant [C.R. 702.8a]), that wouldn't change the rule that you can't play a land, "for any reason"—
A similar situation to your question involves the existing cards Teferi, Mage of Zhalfir and Dryad Arbor. For that, see this thread and this thread.