Currently we one "universal" macro per calling convention (stdcall, etc.).
I found that wrapping procedure call in macro is very convenient, though it requires additional work.
Anyway, we're having rather heavy construction of ["call macro" ->] stdcall -> pushing arguments to stack -> call -> popping arguments to registers -> processing.
It would be nice if we could easily create macros that would pass arguments and preserve registers in a way specific for each particular procedure. I mean something like
Definition:
Usage:
It would be even better if such macro could track usage of registers in the procedure body and automatically generate required push/pop.
However, I don't think such functionality can be implemented in FASM macors without being extremely complex (i.e. slow and buggy)

What problem are you trying to solve with this idea?

You need to define the problems first, then measure how much "damage" those problems are causing. And only then come up with possible solutions. Next step is to weigh up how much "pain" then solutions cause compared to the "damage" caused by the problems. Often the "pain" outweighs the "damage" and makes the solutions pointless. Have you done the comparison yet?

That principle must be implemented by you in the case of an extreme.
You have said to resume registers back: stack used anyway.