Canonical lists and optional arguments

Maybe someone who is more comfortable with the hierarchy could help me.

We introduced this new list type for non-gappy things (see here). And that’s fine, I can use that in some languages sometimes, and it helps.

But I am looking now into possibly just using it always in head-adjunct rules, because, conceptually, it kind of should work across the board I think.

But I am not sure what to do about dropped arguments here. Something like a head-opt-subj rule requires an unexpressed-reg as the subject on the head daughter and that one is non-canonical.

Is there some cross type I could actually use here, must unexpressed also be non-canonical? I am getting lost in the hierarchy a bit.

To summarize, I want a type that means: the argument can be dropped, or we are still looking for it, or it was successfully attached, but it can’t be a gap.