Here's what rhymes with spontaneously.

Consider these alternatives

arise / size decompose / those coalesce / less onstage / stage improvise / size disintegrate / great regenerate / late evaporate / late behave / gave crystallize / size ignite / might magically / mathematically recombine / line arises / prices occur / her germinate / late sprang / an arisen / given disappear / will coalesced / best reappear / will burst / first joyfully / noisily ignites / rights arose / those polymerize / size sporadically / automatically molecules / schools dispersed / first