Also came up with a idea for a Demi-fiend mechanic that plays up Nocturne's customization and Press Turn system, that still has demon summoning elements.
Neutral Special is a joint
Magatama Ingestion + Demon Summon. 3 out of 9~ options appear of joint Magatama+Demon combos. The Magatama and Demon share elemental weaknesses/strengths. For balance purposes they don't necessarily correspond to actual Magatama from Nocturne and simplify most elemental weaknesses to opposites (ie fire/ice, force/elec, light/dark). You can have one at a time and they add the following effects:
- Vulnerability to a specific type of attack; more damage taken, and increases hitstun taken
- Resistance to a specific type of attack; grants superarmor, reduces damage taken and your shield will reflect projectiles of that element
- The demon adds their efforts to your attacks similar to Arsene. They have a weaker attack (for tilts and aerials besides neutral/back air) and a slow attack (for smashes/back air). They don't always gel well with Demi-Fiend's base animation
- Side Special is changed to a spell from the demon
Examples might be
Jack Frost: weak fire, strong ice, quick attack is icebreath with low damage but some hitstun, slow attack is his windup punch with set knockback. Side Special is Bufula, like PK Freeze, focused on immobilizing foes.
Pyro Jack: strong fire, weak ice, quick is fire with decent damage but no hitstun, slow attack is lantern-flailing which sends them up slightly. Side Special is Maragion, which sets a length of stage on fire for an instant, basically free damage.
So on until the major elements (including physical, PSI, etc) are accounted for. Pixie is the default (ie you spawn with her). The trick comes not just from managing weaknesses/strengths, but also taking into account your demon's two moves. For instance, Frost's ice can help with combos on slower aerials, but his slow punch might knock people out of Smash attacks. Demi-fiend is generally slow (think his run speed in Nocturne, that's like Brawl Ganondorf slow) but strong so quicker demon attacks can help by time, prevent him from being zoned out, etc.