Ah throw in inland trade routes, go on. You know you want to. You could put them on a trade menu like you do with the trade posts. You could even upgrade them by adding pilgrims hostels in provinces where there were traditional pilgrimage routes, like Northern Spain or in the Hijaz. Then the longer the line of contigious provinces with trade routes built, the more profitable it will be to build trade posts there
Ah go on. Pretty please?
transposing sea system to land system might be quite possible indeed but that will once again separate land merchant republic (if it ever existed ?) to none merchant ones.