Indeed it takes the regular plural according to Geoffrey Leech’s ”Communicative Grammar of English” saying quote, ..nouns edning in „is” (Greek) : The regular plural occures in „metropils-metropolises”. But again there’re sources that claim that „common usage overrides grammar and commen sense as we have the stress here further away on the second syllable…. Yadda yadda yadda :)
Well, it doesn’t take a grammarian to feel that „metropolii” would sound better to your ear but …
Matrix world of grammar :).That was my new „coinable” notion.:)