* -ly: This suffix turns an adjective into an adverb. Spontaneously means "in a spontaneous manner."
* -ity: This suffix turns an adjective into a noun. Spontaneity means "the quality of being spontaneous."
* -ous: While this is already part of the word, adding it again would create a double suffix and make the word sound awkward.
* -ness: This suffix also turns an adjective into a noun, but it carries a slightly different nuance. Spontaneousness emphasizes the state of being spontaneous, while spontaneity refers to the quality itself.
* -ation: This suffix typically turns verbs into nouns. While "spontaneous" is an adjective, you could use this suffix to create the noun spontanization, which would mean "the act of becoming spontaneous." This is not a common word, but it is grammatically possible.
The best suffix to use will depend on the specific context and the desired meaning.