I'm meaning like, a fangame with them designed around and built in from the ground up - not just thrown in willy nilly, since either it wouldn't be balanced properly or it'd only be a Thing that the player uses unless it's a consideration for the whole game's design an both enemies and allies can take advantage of it.

With TP characters, you have two options: Give them a token amount of SP that functions just as another HP pool, OR make them immune to SP knockouts by virtue of not having any SP. I'd probably prefer the latter since it adds yet another layer of mechanical and strategic depth.

The fact that SP is both less numerous and can thus be removed faster, AND can be healed for cheaper depending on the character (some characters having moves that just serve to restore their SP is still A Thing) is still retained.

You can optionally add moves that are cast from HP as a more common thing to compliment the way SP now functions if so desired.

I personally think it'd add another layer of strategic depth - SP deaths can be quick in both the player's or enemy's favor but SP is also pretty easy to heal for a lot of character (and Joy Mutants if you make it so their turn-wasting moves restore SP a la Desolate Expanse).

(As for what made me think of this: I saw someone complaining about MP death in Star Ocean 3 and went "Coward!" and then "Wait, would that kind of difficulty increase and strategic depth work in a LISA game?")