How will the Red Dragon take off from Mars and get back to earth? All the drawings I've seen just show the capsule on the surface and no rocket to take off again or power the return flight. Could they really build that all into the Dragon?
At this early stage, it is unclear what SpaceX is really planning. They have passed up on the 2018 window, and now planning for the 2020 transfer window. This means they will have two more years to change their plans.
Early suggestions have been to treat the inside as a missile silo with a one or two stage booster to launch from inside the Dragon capsule to return a payload. It is still very early in the planning.
Everything about Red Dragon is in flux, including whether it's doing a sample return at all, as the main purpose is to demonstrate landing. Chances are sample return depends heavily on how SpaceX does in the next few years, and where they focus, as it will be a challenging mission to engineer.
I haven't seen a single proposal to have the capsule itself take off again after landing, only small rockets launched from inside the capsule. This makes sense, as Red Dragon is far more useful staying on the surface than being sacrificed for maybe a few hundred m/s, max. It would also require less re-engineering, as most of the changes would be restricted to the interior and top hatch.
In situ resource utilization (ISRU) to enable lower launch mass and a higher return payload is unlikely, as the Dragon 2 does not use methalox.