I don't make a list of animals that I must have on my island. But, at the beginning of any AC game, I always try to have at least one of each personality type.
In NH, this is necessary for reactions and DIYs. I've already gotten all my reactions but I'm still lacking certain DIYs, so it's still a consideration. (I know you can get all DIYs from bottles. But if I could increase my chances, I'd keep the relevant personality type as well. My two cranky residents have given me their photos and even real art for the museum, but not the bonsai shelf DIY.

)
Even as I play the game, I'm unlikely to develop a "villager goal list". Animals come and go. Sure, I have some I am more fond of than others, and I'm unlikely to let them leave for the forseeable future. But someone will always leave so that someone else can be invited. For me, this gets even more important after a year, when we'd have (very likely) seen everything there is to be seen in the game. I believe AC has such a large roster of animals for a good reason; getting to know the ones we've never met before will keep things fresh and interesting.
I've never seriously contemplated having an island of only one species of animal. I like variety.