At present, the Google Map in TMS Web Core isn't as complete as the VCL and FMX versions. I write software for high altitude balloon tracking, where the balloon, chase teams and fixed listening stations are displayed on a Google map. For Windows (VCL) and Android (FMX) I can move the balloons (markers) and draw paths (polylines) but for the web version I cannot.
So it would be really good for markers to be movable (have the AddMarker procedures return the added marker, and then provide a MoveMarker procedure). I've done this myself by copying the TWebGoogleMaps class into a new unit in my project, renaming it and making the changes I suggested. My JS experience is almost zero but it didn't take long at all.
Also, the addition of PolyLines would be great, as then I can show the path of the balloon so far, and possibly the predicted path.