>>58372730alolan exeggutor absolutely could've fit as a mega; the addition of the dragon type is something a few previous megas had done (charizard x replaced flying/sceptile and ampharos added dragon). it also adds a face, calling back to exeggcute's 5 faces, and elongates the neck, which megas often do (exaggerating a few select features)
alolan golem just gives mega vibes - it adds "facial hair" similar to mega aerodactyl/mega glalie, its limbs simplify as a callback to graveler, while adding the railgun as a new unique feature like mega absol's wings, the things floating around mega steelix, mega sceptile's tail cannon...things like that
add that to the fact megas returned in gen 7 (albeit in a reduced capacity), it wouldn't surprise me if these were designed originally to be megas. i'm not saying this is true, just an observation i noticed when compared to some of the other regional forms which more drastically change type and design
