A Sith that doesn't use the Dark Side would be weird and I don't think it's fundamentally possible to use the powers of the Dark Side for good - at least, not in the long term. There seems to be two ways to be able to use the force effectively. One seems to involve lots of deep breaths and meditation and concentration. The other seems to be harvesting anger, hatred, etc. and using that to control the force. I think of it as similar to that time that Harry tried to use the Crucio curse and it didn't work because he didn't truly have hatred in his heart and truly want to inflict pain on another person. The feelings and emotions that power the Dark Side, when used long-term, are not conducive for good will to man. Given this, I think the film would have to be on Siths as villains. I don't think this is a problem. Sony is making a Sinister Six movie, iirc. I also think we should avoid a tragedy of someone who gets seduced by the Dark Side because we've literally just had that.