Could there be an alternate timeline film in which Doc really did have to wait until sometime in the next century? Christopher Lloyd is still alive and could still play Doc Brown who is finally able to get some gas for the Delorean.
If i did the math right, Doc was Christopher Llyod's real age of 47 in 1955, which means he would've been 77 in 18/1985. The first gas station opened in 1905, in St. Loius. Doc would've been 97, somehow hauling the DeLorean halfway across the country at a frail age.
I mean, I guess it's not impossible. But he would've been waiting a long time, assuming the DeLorean had no mechanic problems after sitting for 20 years. Also, unless Doc is actually supposed to be in his 30's.
In part II Doc said he went to a rejuvenation clinic which essentially reverted his biological age back to his 1955 self. He would end up being the same age as old man doc brown from the first film.