For the Straits feature, I was thinking, maybe there would be this pseudoroad-like feature that connects the centers of two diagonally adjacent water tiles with an invisible line which alters the height map along it, effectively making a cut on the perpendicular land tiles and creating a strait. Maybe this invisible feature could also allow these I can't do it myself though since I have no experience in game graphics design. I think
@merijn_v1's previous experiments with the diagonals would help a lot here.