Familiars are fun. For my games I generally allow a Magic User that wishes to begin play with one to simply to drop a die from their starting 3d6x10 for gold. This die serves double duty as the Familiar’s HP.
Usually they’re treated as fairly mundane animals/pets (unless the Magic User has a First Level Spell Feature that applies to them), maybe a little more clever (smart as a Chimpanzee is generally a good guideline), but usually they’re just a bit of flavor, or potentailly a useful tool that might be useful situationally to solve a very specific problem. Since they’re hard to replace and fragile though, this makes them fairly bad fits as scouts and trap-springers, so I often caution the players to be careful with them.
But sometimes and in some settings it’s more interseting to give them some special abilities.
So here’s a list of One Hundred Familiar Features for inspiration. Perhaps the Magic User uses Magic Reasearch to unlock these secrets (time and money), or maybe they gain new features in play. I suppose you could even have one unlock randomly every few levels the Familiar survives as a tacit encouragement to be careful with those little critters.