I proposed this under the original rules long ago and some people liked it, but it wasn't implemented.

Well, let's if the new rules are more amicable to it.

I was thinking of an optional rule for "adrenal surges" that would be a little like fate rolls but are purely a physical phenomenon that has nothing to do with anything supernatural at all.

They would allow you to add a die to a physical feat or roll, and like fate, get harder to use every time.

The old iron crown system had "adrenal maneuver" as a skill that let you use something like an adrenal rush to improve your physical prowess briefly.

While these got a polite declination under the original rules I think under the new rules making each one cost a stamina point might just balance them out. Also, you lose a stamina point even if the effort at using an adrenal rush fails. Ouch. And no, you cannot combine adrenal rushes and fate boosts.

You could use them for things like adding strength to a blow, adding distance to running, trying to remain conscious, etc, but not for the other things you could use fate for. They would purely affect the player character's physical performance, nothing more.

Some people might think that fate and adrenal surges might be too much, maybe they would be. I'm just offering the idea for consideration.

I think it just overlaps with Fate and is really just a subcase. Since Fate lets you adjust any roll by +1d, and +1d doubles your lifting capacity, the game already has an implementation of what you want. You just want more of it, which would be a house rule for own campaign.