the 6th project skin might be for jax
why would they make such a secret out of it? because i suspect it will be coming with a vu.
jax is one of the original 40, which makes it likely that they already started working on an updated look for him. you might think, why would jax get a vu, when he doesnt look bad? because all of them are getting one at some point and its easy. it seems like one of those "low risk" updates on the scale of ashe, that could happen any time. they dont have to change anything about the general look about the character and might just give him an updated model, some better animations and adding things missing compared to todays standarts like a recall animation. it would be nothing close to the update they would give yorick or poppy, that come tied in with a gameplay update.
jax lore is also currently still tied in with the institute of war. and riot has been phasing it out as part of the world for quite a while. mabye he will get a lore update with a vu, that ties his lore in with yasuo. (hence the same trace numbers).
jax didnt get a skin in quite a while. he would fit into the general style of lone fighters who are apparantly getting a project skin. the helmet on the teaser has 6 eyes. and the way the hair comes out of the helmet looks very much like jax.