Much of the system already uses street-running, so adding more isn't the problem some imagine it to be.
I'm getting tired of saying it, but street-running trams work fine in many cities across Europe but apparently us Brits can't handle signal priorities to make it work.
And if they used green track instead of just continuing tarmac for the joining/leaving street sections like quite a few systems have, it would probably discourage the perennial problem of someone not paying attention and getting stuck! (There are definitely a few bits of the blue route that the road markings could do with a repaint out towards Crystal Peaks). Not to mention keeping inline with the current city tourism marketing as "the green city" and the Castlegate "Grey to Green" scheme.
Another thing I'd quite like to see is replacing the current shelters if they're doing an order for a new route- they're getting quite tired and dilapidated (not to mention Stagecoach/SYPTE's reticence to keep the boards up to date)... I was toying with the idea that something out of weathering steel would look quite good, age well, and pay homage to the industrial heritage of the city. Oh, and more benches would be good- the fact that there's barely any on a large part of the system with current tram frequencies of half an hour (or longer as I discovered the other night) isn't really good when quite a lot of the patronage is elderly!