It's not so much the lack of an idea (I'm sure they could come up with plenty of concepts for a theme), but rather finding something that qualifies for an Ultimate skin.
Ultimates are special because they push the tech of skins forward in some way. Ezreal had an evolving model, Udyr had 4 different models in one champion, Sona had 3 entirely different models coupled with 3 background music tracks that her team could listen to (and using abilities added levels to the song as well). Lux was 10 entirely different skins in one, with the ability to evolve them as the game progressed (and it completely redesigned how skins are coded in the game due to how much data it would need to carry).
They already ran into the problem with GGMF where they weren't really sure what to do next. There's not much more headroom for how they can develop skins, so they reused older things they already developed (hence the lower cost of GGMF). However, we've seen how the community reacted to that (poorly), so Riot is likely holding off on a new ultimate until they can actually come up with some kind of new tech to use.