I definitely think Servo layout can be made useful. There have been no indications that parallel layout doesn't work in practice. Our performance tests have had good results, comparable to the results we saw in Stylo.

I would like to redo the way Servo layout scheduling works, possibly when async/await lands in Rust. I believe that will not only improve performance but also fix a whole bunch of random bugs relating to floats where our scheduler currently just does the wrong thing. (That doesn't mean we have to throw out all of that code or anything like that; it just means I'd like to redo how that works.)

Once you have layout, style, and graphics out of the way, what remains—really, at that point, the difference between Gecko and Servo, aside from embeddability—will mostly be DOM APIs and relatively isolated components such as image decoders. It's an open question how to use Servo DOM API implementations in Gecko. In any case, though, Servo's DOM story may change with the arrival of Alan Jeffrey's Josephine or something like it, and it probably makes sense to revisit this question once we decide how all of that will shake out.

And, personally, I don't know if you consider Pathfinder part of Servo or not, but I'm certainly aiming to get that in.

One thing's for sure: we won't be running out of things to do in Research any time soon. :)