Mr. Red, most of those are already plurals.

gymnasia: proper (Latin) plural of gymnasia
referenda: proper (Latin) plural of referandum
opera: plura is operas
data: proper (Latin) plural of datum, but now often considered to be a word with the same form in both numbers
bacteria: proper (Latin) plural of bacterium
octopussies: improper plural of octopus; proper plural is either octopuses or octopi
vortices: plural of vortex
apices: plural of apex
dice: plural of die
indices: plural of index

And the plural of cafeteria is cafeterias.