What if Dan Aykroyd played Keys instead of Peter Coyote?
Given that Aykroyd had previously worked with Steven Spielberg in 1941 and The Blues Brothers (and later in a blink and you missed it cameo in Indiana Jones and the Temple of Doom) and his real life fixation with aliens and UFOs:
https://www.independent.co.uk/voices/columnists/rebecca-tyrrel/rebecca-tyrrel-dan-aykroyd-believes-aliens-visit-us-because-they-don-t-dance-mick-jagger-7765868.html
https://www.mirror.co.uk/3am/celebrity-news/ghostbusters-dan-ackroyd-says-aliens-16158484
I suppose that if somebody like Dan Aykroyd was in E.T. people would immediately assume that it's supposed to be a comedy. And like Harrison Ford (who apparently plays Eliot's school principal), his presence would've probably been to distracting.