I brought this question up in Studio Blue's stream today. Teal's (and chat's) advice is below.
To review, the question I asked was: How would you craft an RPG battle system? What has been your favorite RPG battle system and why?
Teal's Responses:
https://clips.twitch.tv/BumblingAwkwardHumanSpicyBoy-1ebIGLrAFXLgG5EG
Additional advice from BG and Steel:
https://clips.twitch.tv/PiercingPatientOysterHeyGuys-DMPZJ1FqRK_3aCpZ
Bonus advice if you ever consider trying a 5-party member battle system (which would require plugins):
https://clips.twitch.tv/OriginalMagnificentKleeCclamChamp-93-PIlOPqNuKF8sr
Bonus advice for 5-party member battles, part 2:
https://clips.twitch.tv/TsundereTastyChinchillaDAESuppy-lntGTT_ewg_TqV8b
I hope that gives you some food for thought!