Do you mean, what would it take now, or what would everyone think is an acceptable level of training to allow someone to be a PPL/FI?
If the latter, then I think the greater level of groundschool knowledge is needed. Most PPL's haven't really got the foggiest about a lot of the technical stuff and are just able to scrape the exams in the first place! To be a good teacher, you need significantly more knowledge than the level you are trying to impart, otherwise you'll end up making a hash of it.
The flying side however is a lot easier to deal with. If someone has 500+ hours from a reasonable period of time (not 50 years at 10 hours a year....) then hopefully they should have the required skills to be able to handle most things a student can throw at them.
But, and it's a big one. Most high hour PPL's I've flown with have some pretty atrocious bad habits that would certainly be unacceptable for students. So a great period of "unlearning" maybe required. A far more difficult thing than learning in the first place.
It isn't impossible to fix this and not all have them in the first place, but it is something to be wary of.