I imagine you want to put the OpenGL rendering part inside the maps applications, and changing the server to also serve vector maps ? Would you also want to use OpenGL to render raster map (for the apps not supporting vector) on the server side ?
Seeing the progress made in QtLocation 5.9 (http://blog.qt.io/blog/2017/09/04/qtlocation-5-9/), do you think backporting the whole QtLocation to Qt5.6 would be possible and not too much work, in order to stay as compatible as possible with those future improvements ?
On my side, I am working (albeit a hundred times slower than you) on a stripped down HUD like interface for another target (Odroid C2). So far, I used OSRM (Open Source Routing Machine) as it was the first of all the engines I got to compile (OSM has a lot of dependencies in comparison...), and has an embedded server. I played with its debug vector tile output for some tests (http://project-osrm.org/docs/v5.7.0/api/#tile-service). I decoded them manually with QPainter so far, as they are only containing roads, not the full maps details as MapBox can render), OpenGL is my next target to allow tilting the maps to show the horizon, like you would have in other car navigation systems (like old Here Maps : http://www.geeky-gadgets.com/wp-cont...-here-maps.jpg). You are making me think about how to move this to something that could be shared with Sailfish, despite I would have trouble to keep up with your pace, and not having Silica and other Sailfish specific components...