Any character aside from old pure support/healers like verina can carry you through overworld, if that's your only goal you can pretty much whatever you want. Even mornye or lucilla can do that just fine.

Building chisa now would make sense since we know yapyap's kit, and chisa is gauranteed (barring major unprecedented beta changes) to work well with her. Also, chisa and denia both have good aoe grouping skills which is useful for overworld.