I'm suggesting this :- Aetheric Resonance (Rank I II III IV)

50% chance to obtain a buff named "Aetheric Resonance" when a meld is failed.
Rank of buff correlates to the type of Materia used Grade IV == Aetheric Resonance IV

Aetheric Resonance is lost when a similar buff of different rank is obtained, and or a meld is successful implemented.

Aetheric Resonance stacks up to 15 times, and each buff grants a (100 - current meld chance)/15 * current buff stack % in addition to current meld chance of the similar ranked Materia. (IV only buffs IV chances)


TLDR: What this does: Gives a buff to ensure that Materia melding does not exceed a limit caused by a large number of unfortunately RNG rolls. Given that the last slot is usually 6 -7%, this buff ensures a 100% meld at around 20-30 attempts. A reasonable loss, but not exceedingly obnoxious failures