if basically nothing happens in a section you can just leave it unmapped if you don't have any good ideas for it
ideas are kind of key here, though, so if you have a section that you do think should be mapped, it won't be boring if you come up with some kind of interesting, noticeable, and most importantly,
fitting idea that you play around with throughout the section (for a concrete example, look at the verses (or any part tbh) in HW's
Genryuu Kaiko or the kiais in P_O's
Primal Scene of Japan or everything in happy30's
As the wind blows)
you can also just come up with your own rhythm if the song is too repetitive, it can work really nicely if you structure it in some coherent way
for example, you could do version A of the rhythm in odd-numbered measures and version B of the rhythm in even-numbered measures, which would give you an ABAB rhythmic structure (since songs tend to have phrases that are 4 measures long)
or you could come up with a third version of the rhythm and insert it at the end of the phrase -> ABAC
then if the song's repetitive rhythm keeps going for another phrase, you could come up with yet another version of the rhythm and end up with ABAC ABAD for the whole two phrases
