Yes, there is already tools for making missions for SFC OP.
I found a mission API R2 some days ago.
Doing a mission editor is not a easy task....
1. Imagine a friendly tool that would output a complete mission *.scr. That would require at least a program that could output a complete and functional c++ project automatically, and compile it using the Visual C++ 6 SP5...
2. The other option would be to create an outstanding universal mission, that would read some kind of file, and dynamically deal with any kind of mission. Some kind of interpreter, which file would have to be made from an external tool. Literally, a campaign that could run inside a mission.
Both projects are very complex and time consuming.
And there isn't a easy way to debug applications of that complexity.
Only way to see if it works or not, is to make it, compile, and start that specific mission inside of the game, and see what it does...
It is likely that tiny errors could take forever to debug, even more if those missions are meant to run in a multiplayer campaign.
That is probably why people haven't tried to make it all these years...
But, for what i understood, some people have worked in that area in the past with success.
There must exist some quality missions somewhere.