New region?

Well, Riot mentioned that we only know 3 out of 5 continents of Runeterra... so, its safe to say that someday we will have a new region. When? no idea.

The big question is what kind of region we'd get,as our current regions already cover a lot of different fantasy-types.

A marsh region as you suggested sounds pretty good,as there isn't one in the current lore(though the old one had smaller marsh regions).

Another thing that we don't have is a volcanic region(I always wondered where the magma chamber would be located).

And well personally I'd like to see an Alice-in-wonderland-themed region(though that could be the glade if they keep it canon)

Aren't the Voodoo Lands a big swamp basically? They are ruled by Annie's parents, the Gray Warlock and the Shadow Witch

Wherever Nidalee's from is a swampy sorta place, isn't it?

This comment by Bioluminescence kinda sums this topic up for the time being, IMO:

Planets are big - and there's a reason that I keep talking about a map that shows the three known continents. That's approx 1/6th of Runeterra's surface. Now don't go crazy - there's a lot of ocean out there too, but the chances of there being more than 3 continents on Runeterra is pretty high.

Whether we go and find them is up to the content creators and the needs of the story :)

I mean, there's the Kumungu Jungles. Not officially a region, but at least two champions are affiliated with it in some way (Nidalee and Zyra).

Personally, I'd love to know and explore what's south and west of Shurima. Specifically, all of those jungle territories. Because if this map is anything to go by, then the continent looks huge; clearly on par with Valoran's size and scale.

https://na.leagueoflegends.com/en/featured/piltover/medarda-heirloom

Regarding potential new factions - I think Kumungu has the most promise going for it. Thanks to Ivern's dialogue and some and recent lore updates like the link above, we now know that Kumungu still exists in the retconned lore and is already home to a handful of champions, such as Zyra, Nidalee, Rengar and etc.

However... IMO, these champions never really felt like they were 'interconnected' with League's branching, large-scale narrative, and that might be due to the fact that the region itself was never properly fleshed out into its own fully-fledged faction. All of the current factions have their own narrative themes and arcs going for them (Demacia being an anti-magic and socially strict city-state, Piltover constantly screwing over Zaun to ensure its own successs, Noxus being a relentless, war-mongering empire that values strength and so on,) but the current Kumungu champions don't have some overarching narrative theme that's pulling and tying all of their narrative strings together. It's more like they just... exist to exist. Their narrative arcs just aren't... really going anywhere. At all. Because they aren't given enough substantial grounding that really allows them to continue their development as characters. (Zyra's the biggest red flag in this regard, she's a half-plant hybrid who wants to explore the world... just because? And that's it?) So retconning Kumungu into a brand-new faction would likely fix these issues.

Plus, League has always lacked a jungle-based society & culture niche. Imagine all of the potential new champion or lore-related concepts they could come up with if they actually expand on Kumungu. We could get a cool new Mayan, Aztec. or maybe even a Hindu-based civilization, and that alone would and support several new champion concepts right off the bat. A jungle seems like a perfect place to throw in some Vastaya tribes too. Or they could always go for some really crazy ideas, like more sentient plants, nature spirits, exotic and sentient beasts, dinosaurs, voodoo magic, witch doctors, warrior tribes, Aztec or Mayan-inspired deities, foreign explorers (or foreign exploiters who want to demolish the jungle for natural resources & personal gain) and so on.