OpenRA

Wouldn’t it be nice to make a Openra from Settlers?

Considering that Settlers is more of a tycoon game with RTS elements than actual RTS, I don't expect this happening.

There's https://github.com/freeserf/freeserf however, which only needs AI and combat logic though, the sandbox gameplay already works.
Another option is https://github.com/Pyrdacor/freeserf.net, a rewrite of the above to C# which looks more active now but I haven't playtested that one, so I can't tell more about it's status.