Plans for a
Star Trek movie actually pre-date
Star Wars by a number of years: the first proper work on it began in 1975. The movie was actually cancelled in order to make
Phase II in 1977 (in response to the hype from
Star Wars: the execs thought it would steal all their thunder & got cold feet) before being un-cancelled that same year (in response to the success of
Close Encounters of the Third Kind, which showed that there could be more than one science-fiction blockbuster in the world). If I may give a shameless plug, my TL "
An Alternate Rise of the Blockbuster" explores in one update what could have happened if
TMP hadn't been temporarily cancelled in 1977.
Anyway, back to the actual thread topic. Among other things, it's entirely possible that Leonard Nimoy would leave the show. By the end of it in OTL his relationship with Roddenberry et al had seriously deteriorated and it never really recovered.