Is your feature request related to a problem? Please describe.
My problem is, that I 'm making irregularly shaped aquarium schematic, but I want to make it a lukewarm ocean so it will spawn sealife on it's own (so I don't have to worry about refilling it should they despawn). With the biome brush tool, it is very hard to do precise work and I don't want the biome to spill into my house, so i find myself constantly trying to make selections and logging in and out to check it's progress and it's killing me. This may seem niche, BUT I think it'd be useful for things like wanting all the grass blocks to be treated like a flower forest so they grow flowers, or wanting a spawner to be a certain biome.

Describe the solution you'd like
I want the selection command to be able to create selections within a rough area based off a material. for instance, all water within my home being included in lukewarm Ocean biome, exactly like the replace command but a general selection (or at least with biomes)

Describe alternatives you've considered
While I would prefer this edit for the selection tool, just having it purely for /setbiomes is fine with me. I could just keep trying to keep trying to get better at making selections (I am new at worldedit and sometimes getting precise selections is trial and error for me)

Additional context

As selections need to be contiguous space, I'm not sure this would work well.

You may find using masks fits your needs better

Of note is that masks don't affect biome placement at the moment, though that is currently a feature for being able to paste biomes without blocks. Not sure what the best solution is.

Thank you for suggesting masks (I was never sure how to use them but now I think i've got it down). I was able to use a global mask on set biome, and while there was some spillover it was more or less accurate, though for some reason when i copy and paste, the -b tag isn't copying biomes so i can't save it unfortunately but really helpful nonetheless