There is one downside: the time spent adding new stuff for the modders is the time not spent adding new stuff for everyone else. I guess that some of the suggestions could be used by most modders, but there's really no reason to spend hours coding a succession law that will only be used in a single obscure mod that will be forgotten in a month.
Yeah, to be realistic, I don't think it makes sense to add any new succession laws that won't be used in vanilla. The only thing that might make sense would be to make the whole succession system more scriptable, if that would also simplify adding new vanilla laws and fixing bugs in the old ones.