I prefer them to be actual creatures, but that includes fey and fiends IMO. I don't want fey/fiends/etc. in "animal form". I loved familiars in AD&D, where they were a blessing and a curse depending on what happened.
I think making them a "bit" harder to kill later on makes sense if they've been with you from the beginning, but not too much.
They should definitely have personalities of their own instead of just being totally agreeable to the caster.
Here is a revised 5E Find Familiar spells we've used for nearly two years:
View attachment 150954
View attachment 150955