I'm not up to speed on the current open-source SGF editors, because I mostly use MultiGo or SmartGo, but one feature I've wanted for some time is the ability to extract game variations as problems quickly. For example, starting at a particular move, consider that as the starting board position and all of the variations after it are part of the problem. I'm okay marking the correct variations manually. I think GoWrite2 can almost do this, but it's more designed for printed diagrams and I want something I can use to save SGF for input into other drilling programs like uliGo.
I would consider adding this feature myself to an existing code base, but I'm not going to write an SGF editor just to have that one feature.
