What exactly is the worst thing that can happen to my foot from plantar fasciitis?

Plantar fasaciitis : You could get a tear in the fascia and or develope chronic pain in the arch that prevents you from walking without pain. Its easy to treat in most cases . I would see a podiatrist.

Pain, rupture.: Usually it's just a lasting, chronic pain if left untreated, but very rarely the plantar fascia can actually rupture, which causes major problems in your arch, as you might imagine.