Sunny D is the beverage of choice in Lordran, acting as a consumable with a set number of uses. Basically, it's a heal you can pop at any time, at the expense of making yourself highly vulnerable while doing it; for about a second, you cannot cancel the drinking animation, and can only move very slowly while doing so. In the original Dark Souls, you can't move at all. Finally, in Dark Souls 3, you can occasionally regain a swig by performing well in combat - a mechanic that could reward skilled play in Smash.
In this way, Estus could act as a sort of X-factor, wherein managing it well could be the difference between winning and losing. I'd design it such that you get three charges per game - one for each stock. But, each one would heal an impressive 20-30%, depending on balancing needs. Furthermore, you could possibly regain a charge by getting enough hits in, surviving long enough, or some other metric to reward skillful play and give the player a sort of meta-goal beyond just KO-ing their opponent. A Dark Souls character would be slow, with sluggish attacks and a bad disadvantage state, so they should be built with the expectation of taking a lot of damage. Estus could be the solution, offering a clutch way to undo some of the hits you take at the perfect time. This could be a really interesting mechanic that gives a Souls character a unique flavor unlike anyone else so far.