Goal: Consider how adpositions and case will interact
Note: You may want a preferred order for multiple adposition phrases.
Tip: Strict word order can correlate with strict phrase order.
Work focus: Organize/Plan/Structure
If you have a case-marking language, you need to consider how your new adpositions will interact with those cases. Common lexical sources for adpositions include transitive verbs and nouns. If an adposition comes from a transitive verb source, the noun phrase that occurs with the adposition will likely occur in the accusative (or absolutive) case. If an adposition comes from a noun source, it may require the noun phrase occurring with the adposition to be in the genitive (or possessive) case.
Other kinds of verbs may be lexical sources for adpositions, so other cases may make more logical sense for marking the noun phrases that occur with an adposition, such as the dative case or the locative case. It really depends on the sources you chose to use and the case-marking system you have in place.
Regardless of whether your language has case-marking, you should also consider how adposition phrases will be ordered within a larger structure. You may want to have a preferred order when multiple adposition phrases occur one after another. For instance, if you wanted to say something like “I swim in the lake on sunny days.” That structure has two adposition phrases: “in the lake” and “on sunny days.” When such situations occur, do you want your language to have a preferred ordering of phrase information? You can set a preferred order by the type of information conveyed in the phrase, such as selecting an order of place before time before instrument before manner. In such a system “I swim in the lake on sunny days” would be well formed while “I swim on sunny days in the lake” would not be preferred (i.e. they would sound a bit awkwardly worded to the speakers of your language).
If you decided not to have any case-marking but instead to rely on strict word order, you might also want to apply stricter word order to other areas, including the ordering of adposition phrases.