New maps and worlds are able to be done in GW2's engine.
New professions and skills are still doable here, though whether it's in GW2 or a hypothetical GW3, it's a potential balance nightmare.
Housing is able to be done in GW2. While it would be nice if Anet would update home instances to allow for the guild hall decoration system, it would be far easier to create player housing by using the guild hall system for dedicated housing instances.
New PvP battlegrounds and different modes are also completely able to be done in GW2's engine.
Now, a new graphics engine could be a benefit to GW3, though GW2 looks fine at the moment with its painting-like art style. If GW3 were going for a super realistic appearance, it would absolutely require a new graphics engine, though we'd also run into issues of whether or not the current population of GW2 players would be able to join in GW3 without investing in a beefier system. A common trait among successful MMOs is that they are designed to be able to run on a wide variety of hardware specs, as only targeting a demographic who has high-end machines is an excellent way to produce an MMO that fails to thrive.