Something like creating multiple LightMix elements and selecting from which LightSelect elements will each of these be composed?
I imagine it like this:
the multiple maps from render setup > scene environment get to be on the top of the list of lightmix like they are now, but with a division line under where the regular scene lights are listed. Also enabling one would disable the other hdri of the multiple maps category - because in most scenarios one would want only one hdri or corona sun+sky illuminating the scene. But for cases when additive hdri lighting is needed double-clicking the tickbox will enable it without disabling the other hdri(s) or sun(s).
Corona sun(s) shall be in the top of the list where hdri(s) are because they are being major "players" of the lighting a scene in most cases. Also, corona sun(s) shall have as a child object their respective corona skies that are in the multiple maps of the environment. Ticking corona sun will automatically enable it's sky, but if one wants to just enable the sun or just the sky they open the child object where sky can be disabled. Also enabling one sun shall disable other(s) and their sky(ies) - for cases when one does a 10hour sun study - it takes 2 additional clicks to disable sun, sky, enable other sun, sky.
A quick mock-up of how it could be is attached. Also maybe some of the light mix elements can get custom user input category: living room, kitchen, etc. So one could get this drop-down categories like the "other" category in the mock-up below.
P.S. Also observed that latest Corona Image Editor is slower(sometimes 1-2sec wait to see the effect) on enabling/disabling lights in lightmix. The same lightmix "play" directly in framebuffer is much smoother and snappier.
