Currently BlueMap supports orthographic view and calls it "flat". It is locked to only allow viewing from the top. I see no reason why it is locked. I'd love to be able to view it at an angle, so that it looks similar to Overviewer.

The reason it's locked is, that unlocking it creates a big set of issues with the cameras near/far planes, controls and map tile-loading. It doesn't work easily. Currently this is not planned to be added.

Also please read the contributing guidelines before creating issues. This is the wrong place for feature requests, especially if they are just a couple of words long.