The game of Magic: The Gathering has many a planes, though some have been seen on not more than a handful of cards.
With Bloomburrow soon to be the newest plane to have its own expansion, that's one more plane down with many more to go that have been everywhere from fleeting glimpses to one offhand mention in a novel from the 90s.
That said, a few have been fleshed out enough to have been mentioned in a few cards and perhaps be set for the next unused plane to have their own expansion. In fact, just like the Storm Scale for mechanics, Magic has a scale for revisiting lands: The Rabiah Scale. It's named after Rabiah, a plane used once (retroactively) in Arabian Nights back in the mid-90s, then never used again.
But as for locations mentioned but never used, there isn't really a scale for that, despite dozens of these still unused lands. There's Gargantikar, a plane that is a giant jungle filled with huge insects. More fleshed out is Muraganda, a similar ancient, nature based forest land filled with dinosaur creatures. However, thanks to the main creatures being a 10 on the Beeble Scale, yet another scale but for creature use, that will be a huge maybe. Although they also had a few cards in the past pop up.
Far more likely, thanks to Bloomburrow, is Belenon. This plane, kind of based on wind, has mostly animal creates like Rhox (rhinos), Loxodon (elephants), and Aven (birds). Considering that the few cards we have seen with it are in the midst of battles, and being animal based (kinda), this could be a future follow of Bloomburrow.
Also slightly more possible is Segovia. It's also a place that hasn't had an expansion to be on the Rabiah Scale. The plane, which is an 8, is a plane where everything is at a 1:100 miniature size. But then there is also the really good chance of Mongseng. Introduced in Planechaser as a likely future plane, the land is based on Chinese warrior culture. It was listed as an influence for Khans of Tarkir, which quasi-replaced it after some alterations and retconning. It is still technically distinct and can still be it's own expansion, but like all the others on the list, there is enough.
All being said, Magic has a ton of options through existing planes, and like many others, can just create more as they go. Although post-Bloomburrow, they have some options at the ready it seems.
