I'd like it if intelligent creatures (not just sentients, but anything with an actual working brain) had the ability to choose themselves an area that they consider their "home". Pets with owners would be likely to pick the same room as their owner, while other Fortress pets might pick the dining room (dogs), or anywhere with low traffic and a lot of vermin (cats).
I'd be nice if we could build quarters for domestic animals (barns) to use, also, and (to a limited degree?) designate that specific area as their 'home'.
Cowardly or untrained animals might choose to flee and hide in their home area, during a seige, or even during a caravan (they might view them as similar events). Brave animals, and animals with progeny (or just animals that have lived there for a long time), might defend their homes to the death, and guard/war animals might defend their *owner's* home to the death--or just the dining room, where they get all those delicious scraps of food.
Dwarfs and other sentients might also have a sense of home, which they might then feel compelled to defend, even when not in squad-mode.
Animals unattached to Fortresses could have 'home areas' as well, fleeing to/defending a particular tree, rock, cave, bend in the river, etc. while others (herd animals, and domestic animals without a designated "barn" area) might attach themselves to several of the oldest members of the herd, instead.
Setting something like this up might make ranching/managing animals a little bit more intuitive, and possibly easier, in several instances.
Some animals might also be trained to recognise dwarfs (and other sentients) as their leaders/owners, with a varying degree of recognition for their owners' friends/acquaintances, with different familiarity leading to different reactions.
Animals could return home at the end of their diurnal/nocturnal cycle, store food/hide things there, bear young there, and retreat there to heal wounds/illnesses.
This could differ from territorialism:
Animals might have a greater or lesser sense of territorialism, depending on species(bees)/gender(bull elephants, or just certain types of bulls), so their reaction to an invasion of their "home" may be more or less severe, but I think it could be a good idea to keep 'territorialism' and 'sense-of-home' as separate traits, since not every animal species will automatically fight to the death over the loss of a "home", and not every animal species needs a strong sense of "home" to have an excuse to attack beings encroaching into "their territory".
Strongly territorial animals might retreat to their homes to die, however, while ones who are weakly territorial, but more strongly home-oriented, might specificially instinctively travel away from their homes to die, in order to not pollute an area their young might also inhabit.
Some animals, in future updates, might be pragmatic enough to flee from their home if it's on fire, while others might have to be physically dragged out of burning buildings, refusing to leave even if terrified of the flames (I'm thinking of horses specifically).