My guess is it would be similar to what it is today, specifically in that the diverse landscape, including the three Andean mountain ranges that run through it,

My guess is it would be similar to what it is today, specifically in that the diverse landscape, including the three Andean mountain ranges that run through it, helped isolate regional Colombian cultures (and in isolation they grew stronger). Meaning we'd have a regional, culturally-diverse landscape, with Muisca, Carib, Wayuu and other peoples spread out through the land. As for it being mostly monolingual under the imagined circumstance, I can't say, but it's possible Gran Colombia (current-day Colombia, Venezuela, Ecuador, Peru, Panama, etc.) would have naturally formed and, under it, a monolingual pidgin.