I've been making a tonal language where there are six contour tone morphemes. The three that end on falling tones mark verb forms/adjectives and the three that end on rising ones mark noun forms. I am having problems with coming up with prepositions and other words that are neither verbs/adjectives nor nouns. I was thinking they could have consonants as syllable nuclei instead of the tonal vowels, but that makes the words hard to say, and there are only a limited number of words I can come up with in this way. Do you have any suggestions?