So, some random hypothesizing here.

One of the things that keeps coming up with Fate is how very central the action economy is to the conflicts, and how that impacts things, how it makes single baddies very tough to do, etc.

What if you decoupled the "number of participants" from the "number of actions a side gets"?

I'd probably stick the default at Actions = #PCs.  So if you were fighting a single bad guy with four PCs, you'd have one action each, and the bad guy would get four.

There's obviously some areas that would need to be thought through with this - what if there's more NPCs than PCs, for one.

This might seem to remove any advantage of outnumbering an opponent - but does it, really?  That single opponent is still limited by their single stress/consequence track, and the people outnumbering them could still theoretically get +1 to assist in actions.

I'm not convinced this is a good idea, but it might spark an actual good idea from someone.