I've got a suggestion, pet leveling. Basically, what I envision is a pet advancement tree similar to what we use for EDs. Classes who get pet leveling anyway (ranger, druid, arti) use the new system but proceed as usual. However, as a multiclass into any of those three classes, you get a new feat that you can select, call it Animal Training for Ranger/Druid and something artificer-y for the Arti. What that feat does is unlocks the ability to upgrade your pets like pure versions of the relevant classes with enhancements etc. Basically costing a feat + enhancement points to keep the cool sidekick useful throughout the game. Maybe give them (pets in general) epic tiers as well but force those pets to also cost a party space like a hireling, making them also unavailable during a raid. It'd be a nice boon for solo play and work well with campaign flavor (though not technically by the books rules-wise).