someone, fantomeblacktree, already did this; its development thread ended up getting removed. id advise against developing something and setting it free to the community, cause youll just end up wasting time.
i already made it so u can use layers, copy/ paste /reuse objects (so say u draw a tre you can reuse it later in the track), erase and draw lines + scenery, just got to work on mobing the editor around + Zoom
C++ is powerful, but it's harder for this type of thing and javascript can run right in the browser. If you end up doing it, just do it for personal use, because there will probably be trouble if you make it public.
Copy and paste has been brought up many times, the community voted against it as it contributes to spam.
if you make another editor, Kano will shut it down for copyright reasons. a cool editor with teleporters, no-die mode was shut down lucky i downloaded an offline version