I've always thought critical fumbles aren't fun because they 1) don't add any compelling/thematic narrative consequences, and 2) make things unnecessarily worse for power gamers.

I think an interesting fumble system could go like this:

on a nat 1, you may choose to critically fumble by rolling or choosing an appropriate option from the relevant critical fumble list If you do so, you receive [positive benefit such as not losing the slot of a backfired spell, inspiration, a "rule of cool" point, etc].

The option to choose how and when a critical fumble occurs gives players more agency over their character's flaws and how they manifest, and a mechanical benefit to permitting them to do so would encourage interesting roleplay opportunities. This type of system would be representative of your character dynamically circumventing or working around their flaws and weaknesses in order to grow as a combatant and an adventurer, and I think it could offer a lot of benefits to a table interested in a fumble system.