Horizon or Valk? (Who is the Season 7 Legend?)?

There is no definite answer to this question. The only answer is that you should not be surprised if either of these characters become Legends. However I have a theory which draws upon various sources.

So Horizon (a.k.a Mary Somers) seems a little off to me. She did mention something about her coming to the Games, but you have to remember: Forge was supposed to come to the Games as well. In addition, Horizon has broken Respawn's usual plan of revealing Legends; legends are never revealed a month early. It's entirely possible Horizon is the Season 7 Legend, but again, remember that giant leak with the Legends until Season 10+? Horizon was shown as the Season 10 Legend. 

We know very little about the character named Valk. The first time we heard of Valk was on the leaked sheet which showed the Legends from Season 3-8 accurately. Crypto, Revenant, Loba and Rampart are all on that sheet, and so are Valk and Blisk. We also have datamined abilities for Valk, but not much for Horizon.

My theory is that Horizon will have a similar introduction to Forge. It's plausible to say Horizon is currently situated on the moon, as from her surroundings in the clips. Something will go wrong and Horizon will be replaced by Valk, but unlike Forge, Horizon will return in Season 10.

It's also possible Valk attempts to kill Horizon, but that wouldn't make sense as the Syndicate wouldn't willingly allow someone as a Legend if they "killed" an upcoming one. It was different with Revenant because Hammond and the Syndicate feared Revenant, therefore they used the Games as an opportunity to divert his attention and contain him. Unless Valk is similar, this theory is probably incorrect.