I am aware that the plural of metropolis is metropolises, but to me it sounds stilted and to be honest I cannot recall ever hearing it used.
Is there an irregular plural of metropolis that would be in a less formal register. Any common mistaken pluralizations?
Metropolis is originally a Greek word, but comes to English through Latin. The Latin plural is metropoles (presumably pronounced with /iːz/ instead of /ɪs/). The Greek plural is metropoleis. If you look at a Google Ngram, metropolises is used most of the time, but metropoles is not uncommon. Metropoleis is used very rarely, and when used, it mainly refers to ancient Greek city-states.
Wiktionary and some dictionaries suggest metropolises and metropoleis as the plural, but I would strongly advise against using metropoleis (unless you're talking about ancient Greek city-states); Greek plurals are rarely used in English (see octopodes). But if you want to use metropoles rather than metropolises, you won't be alone.
Metropolises is the plural of metropolis, and for what I know, there aren't other plurals for that word.
Looking for metropolises at the Corpus of Contemporary American English and the British National Corpus, I get the following data:
(Corpus of Contemporary American English doesn't report any data under "non-academic" or "miscellaneous"; the scale is logarithmic.)
The higher number of times the word is found in the CoCA corpus is 31, which means a ratio of 0.38 per million.
Metropolises is used in sentences like the following:
I would refine this further to suggest that when experiences in small towns, ports, commercial cities, and industrial metropolises are compared, similarities far outweigh divergences across the country and that a broadly defined "Jewishkeit," or mixture of Jewish tradition, custom, values, and historical experience, exerted substantial influence over behavior.
According to dictionary.reference.com the plural for metropolis is "metropolises".
