Heya,
first off, I've done a quick search but couldn't find anything but old announcements and ideas for familiar / ranger pet mods, which seemingly never got finished.
If you know of any mod that already does what I intend to do, please let me know and spare me the unnecessary effort.
So the basic idea is pretty simple: Allow rangers to choose one of a wide range of animal companions that work more or less exactly like 'mage familiars' work right now, but are a little more useful/durable. (They probably won't go into your backpack and neither will they die permanently, but I guess that's not really relevant right now.. )
I haven't yet looked to much into familiars, but it seems there is a lot of hard coded stuff involved.
For example, simply making a .cre global via makeglobal() and setting it to the familiar allegiance category won't allow it to switch areas with the party.
So unless I'm missing something here, I think the best way is probably to 'override' (new script, new dialogue, new stat items ...) one of the standard familiars as soon as it is spawned via the findfamiliar effect. (Note: I'm not planing to override the familiar .cre file, but just the specific familiar that is spawned for the ranger. That way, the standard 'Find Familiar' spell would stay untouched for mages / sorcerers.)
I've posted this in the Modding Discussion instead of the Ideas forum, since it is more of an inquiry than a simple idea thread.