I definitely think Dark Star
could be good. Been awhile since he got a "real" skin, he is being played pretty actively, in a good spot(Vel'Koz main speaking), and it could definitely fit thematically. As long as they focus on making the particles unique and far from his base skin's then it'd be pretty sweet. Right now Arclight is Vel's best skin, and in my opinion adding another skin could really give a lot of sales. I'd buy it straight out. Of course, I'd also buy a chroma for Arclight if they had some. Seeing Vel'Koz split a star in half or whatever his Q would be... man I'd love to see that.
Overall it makes sense.
Doesn't have a lot of skins already = CHECK
In a good spot = CHECK
Played rather often = CHECK
Fits the champion's design = CHECK
Personally Kog'Maw already has a lot of skins, and even has his Gemstone exclusive skin, so he can wait a bit.
Alternatively Rek'Sai ALSO is int he same boat, but has waiting MUCH longer for a new skin. Only having 1 new skin since release, which now only Neeko and Sylas have less skins than Rek'Sai but will probably surpass Rek'Sai in the near future.