Riot's own Universe database lists a race for exactly thirteen of League's 173 champions: eight Vastaya and five Darkin. The other 160 have no race field at all, and most of them are human. That one data point does more to answer the playable-races question than any leak, because it shows where Riot itself draws the line between a species and a condition.
Nothing below is confirmed. Riot has said nothing about character creation for the Runeterra MMO, before or after the 2024 reset. What follows is a reading of the lore Riot has published, which is the only evidence there is, and the argument leans on a distinction that most speculation skips: some of the beings people want to play are races, and some are things that happened to humans.
The near-certain: humans
Runeterra's population is human by an overwhelming margin, and so is League's roster. Demacians, Noxians, Ionians, Freljordians, Shurimans, Piltovans, Zaunites and Bilgewater's crews are all, in the main, people. Every region Riot has ever named as a destination for the MMO is a human region first.
The interesting question is not whether humans are playable but how they are divided. Runeterra's humans are not interchangeable. A Demacian raised under petricite, the stone that drinks magic, has a different relationship to sorcery than an Ionian who grew up beside the spirit realm. If the MMO offers "human" as one option, it throws away the most usable cultural texture in the setting. If it offers Demacian, Noxian, Ionian and so on as distinct choices, it gets faction identity for free. Riot's regions article on Universe already reads like a character-creation screen.
The near-certain: Yordles
Yordles are the second easiest call, for two reasons.
The first is popularity. Teemo, Tristana, Lulu, Veigar, Poppy, Ziggs, Heimerdinger, Kennen, Kled, Gnar, Vex, Corki and Rumble are among League's most played and most merchandised characters. A Runeterra MMO without Yordles would be leaving its own mascot outside the door.
The second is the lore, which makes them a species in a way nothing else on this list quite is. Yordles are spirits of Bandle City, a realm that overlaps the material world without being part of it. They are, in Riot's telling, immortal, magical, and able to take a physical form among mortals. They also have an entire faction to themselves: Bandle City is one of the thirteen regions on Riot's map, with its own roster of champions and, in Legends of Runeterra, more illustrated cards than any other region. A race with a homeland, a culture and a population is a playable race. The design problem is scale and animation, not canon.
The likely: Vastaya
Vastaya are the third group, and the one where the lore argument is strongest and the production argument weakest.
Riot's Universe entry describes them as a chimeric people, descended from the Vastayashai'rei, mortals who took the magic of the spirit realm into themselves and were changed by it. Their descendants carry a diluted version of that magic and a visible share of animal form. Ahri is fox-like. Rengar is feline. Xayah and Rakan are birds. Nami is of the sea. Wukong is a simian. Neeko can become anything. Aurora, the most recent, is a hare of the Freljord.
That variety is the case for and the case against. For: eight officially listed Vastaya span five regions and half a dozen body plans, which is a character-creation system waiting to be built. Against: those body plans share almost nothing. A studio can build one human rig and one Yordle rig and cover most of Runeterra. Building Vastaya means building several rigs for one race. Riot can afford that. Whether it chooses to at launch is a scheduling question, and scheduling questions get answered late.
Our read is that Vastaya are likely at launch, in a limited number of lineages, with more added over time. That is how every MMO with a complex race has handled it.
The out: Darkin, and why
This is the argument that settles most of the rest of the list.
The Darkin were not born Darkin. Aatrox's official biography opens with it: "Once honored defenders of Shurima against the Void, Aatrox and his brethren would eventually become an even greater threat to Runeterra." They were Ascended, god-warriors made by Shurima's Sun Disc, who were corrupted, defeated, and imprisoned inside weapons. Aatrox is a sword. Rhaast, the presence inside Kayn, is a scythe. Varus was two hunters fused with a bow. Naafiri is a dagger that runs a pack of hounds.
A Darkin is not a species. It is what happened to a handful of specific people, and every one of them is a named character with a plot. There are five in the entire canon. You cannot roll a sixth without either contradicting the lore, in which the Darkin are a known and finite set of horrors, or making a player the equal of Aatrox at level one. Neither works. The Darkin are boss material, and probably the best boss material in the setting, which is a different article.
The out, for the same reason: Ascended and Iceborn
The Ascended are the same case one step earlier. Nasus, Renekton and Azir were humans raised by the Sun Disc into something more. Xerath was a magus who seized the ritual. Ascension is an event, a rare one, performed by a specific artefact in a specific city that spent centuries buried in sand. A playable Ascended would mean the Sun Disc is a character creator, and Shurima's whole tragedy is that it was not.
The Iceborn look more promising and are not. They are Freljordians marked by True Ice, the ice that does not melt and burns anyone unfit to hold it. Sejuani and Ashe carry it. Lissandra, one of the Three Sisters who founded the lineage, is its source. Iceborn are humans with an inheritance, which makes "Iceborn" a good origin story for a Freljordian human character and a poor basis for a separate race. The distinction sounds pedantic until you try to design a character-creation screen around it.
The maybe list
A few groups are genuine species and are not obviously in or out.
Minotaurs exist as a people; Alistar is one. Trolls are a people; Trundle is one. The Brackern are a nearly extinct crystal species; Skarner may be the last. Celestials and the Aspects of Targon are entities that inhabit hosts, which makes them a mechanic rather than a race. The creatures of the Void are a hierarchy of things that want the world unmade, and Kai'Sa, the one human who wears the Void as a suit, is the exception that proves how far Riot keeps them from playability.
Any of the first three could appear as a race later. None of them has the population, the culture or the character roster to justify the cost at launch, and MMOs that launch with a large number of thinly written races tend to spend years fixing it.
What a launch roster probably looks like
Labelled as speculation: humans, divided by region, plus Yordles, plus a limited set of Vastaya. Three races, one of them subdivided, which is a smaller list than World of Warcraft launched with in 2004 and a larger one than most of Runeterra's fiction needs.
Everything past that depends on decisions Riot has not announced, starting with the one that matters most: whether players are ordinary people of Runeterra or something closer to the champions themselves. We take that question up in everything we actually know, where it sits on the list of things Riot has never addressed.
What we do not know
Whether the MMO has races at all in the traditional sense. Riot's reset promised something "different enough", and a fixed race list is one of the genre conventions it could have dropped.
Whether Vastaya lineages are treated as one race or several.
Whether a human character's region is a cosmetic choice or a faction with consequences. The PvP implications are large either way.
Whether Riot's 2023 decision to unify the canon with Arcane constrains any of this. Arcane has so far shown humans and, briefly, one Yordle. The unified canon may need Vastaya on screen before the MMO can ask players to be one.
If you are new to the world and want the ground these arguments stand on, start with what is Runeterra? and then where to start with Runeterra, from zero.