The extension idea has flitted around for pretty much as long as I lived in South Yorkshire, and that was nearly two decades. Proposals to extend to Maltby, Ravenfield, Dore, Millhouses etc.
Dore and Millhouses is a no IMO as Abbeydale Road is already congested enough, so unless they found a place to build a new alignment, or ran a tram train when Dore is eventually redoubled, then I can't see that being feasible.
Fulwood already has First serving it every ten minutes, so that would be a waste, and some of the gradients around that area leave buses struggling, never mind metal on metal.
An extension is possible yes definitely, and once the future of Supertram is secured then it may be a possibility. But there has to be catalyst(s) in other areas, some of which I mentioned above.