Gnar Skin

Depends on how popular he is, that creates the demand for skins to come sooner after all. Like Vi's Officer skin came way faster than it should have and since then several newer champs picked up skins all due to their popularity.

Some champions definitely get new skins faster than others. I'm not sure it's really all about popularity though. It's possible that Vi has gotten 2 skins relatively quickly because she is very popular. But it's also possible that she got skins quickly because both those skins were pretty straightforward and easy to design. I mean, Kha is also insanely popular, and it took them forever to release a skin for him, because Guardian of the Sands was simply way harder to design and produce than something like officer Vi (the theme is less obvious, Riot had to model all of his evolutions, he has more particles to change, etc.)

Making a skin for Gnar is twice as hard as making a Vi skin. Gnar has 2 forms they have to design, as well as a projectile to play with.

I'd be surprised (but very excited) if Gnar got a new skin anytime soon.