You do know your asking for a whole new game here don't you, what your asking for is an entire game being released as an expansion.
Doing all this alone Could probably be a stand alone game let alone having it as an expansion to elite.
I think some people are wishing for too much, these are £20 expansions not whole new games at £39.99!
I don't think it's as bad as you make out here. I can see the following as the progression for planetary landings, building on existing game mechanics and going from what - to me - seems the simplest to the most complex.
First off you'd want to start with atmospheric flight and ground facilities that you can land at. No getting out of your seat still, so exactly like the landings and atmo flight in Frontier and First Encounters. This uses the systems already built into the game - a supercruise like transition from planetary orbit to in atmosphere with a 're-entry' type loading screen for the instance loading. Flight mechanics would be mostly unchanged, but top speed might be capped differently by ship to account for gravity, mass and air resistance. Surface structures and details can be procedurally generated using the existing algorithms with minor modifications for surface topology and geography.
Could even simplify this stage by starting with airless rocks, barren planets and asteroids so you don't even need to worruy about extreme gravity or air resistance physics.
Ground vehicles could come next. we already have fully rendered 3D cockpits, a buggy 'cockpit' isn't that much different. The control scheme would be similar but operate based on the fact that there's no perpendicular movement, lateral thrust or roll, just acceleration, deceleration and yaw effectively. Ground scanning could be done similarly to the existing Discovery Scanner 'honk' mechanic, as well as detailed scans by getting within range of an object - exactly as we scan planets as is. Could even use a similar piece of functionality to the existing cargo scoop mechanics to collect objects for sale to Universal Cartographics or the Commodities markets.
Then you can start thinking about first person walking around. That's the kicker as the mechanics of that are potentially sufficiently divorced from the control scheme where we operate based on velocity and relative movement. However, once you've got the collision physics and other thigns in place for a solid piece of ground beneath the player then you can build the rest up from there. Again, as with atmospheric flight, start with contained areas. Inside your ship, then within a station, and then potentially full free-roam on planets.
If it's all done in stages you could build it one block at a time. Staged development seems to be the way Frontier like to do things, and with good reason. They have the core of a game at the moment, they've added structures to add to that with the back-end mission system revamp. Then they can add more variety late. The same can be said of any feature to come. Start with a baseline and build up from there.
That's the point of a long-term development cycle. Build the basics, test it, release it, fix it if needed, then build the next block to put on top while your users have the basic thing to play with in the mean time.