There's got to be some threshold, though. I agree that new faces need to be able to get on the stage, but it can't just be anyone who wants to. Debates with too many candidates are chaotic.
Local elections for me are based on a minimum number of signatures. It'd be nice if presidential candidates had to go and get some number of physical signatures themselves. It'd be a good way to force them to spend at least a little bit of time actually meeting people. Though this would probably get gamed somehow.
They do to get on the ballot, though I don't think they need to get those themselves physically. It's not exactly practical at the scale of the POTUS due to the number of states and population, since each state party has their own requirements.
I think the best way to get new faces would be a solely public funded campaign process combined with something like ranked choice voting. The current levers are controlled by too few now for any minor rule changes to move the needle.