There's 2 categories of regression headcanons
You fully believe this character is or could be a regressor/cg/flip/something. There's bits in canon that you can point to and say that's why and there's reasons why they would specificially regress or be a caregiver! A headcanon or heartcanon.
For sillies. Feasibly, you could write ANY character to be part of the community! You don't actually think it would fit but you're either imagining it because you're fond of the character and there's just something about it or you're going through all the characters in your head an applying babification to them. I call these "regressorland headcanons" where you prioritise the regression over being true to canon.
Both are great Both are entertaining! But gosh it can be hard to get yourself to answer "do I actually think this would fit or am I just doing this for the sake of it/doing what i think could possibly apply"