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The prepositions come under the heading of lexicon, so there isn't much to say about them here in terms of grammar, other than the proposal that certain of them be allowed to function as conjunctions (e.g. vor before, efter after, viil while/during, til until, sid since).
But in terms of derivation, it will have to be decided how adjectives and other prepositions are formed. For example, shall we imitate the English usage of to for motion, as in to + in = into?