I think we should stay imaginative about how these things would expand the scope of the Elite universe, not just see them as bolt-ons. The gameplay of Elite comes in to its own when multiple elements combine to create experiences which will literally never be repeated. With more types of gameplay (exploration,
prospecting etc.) - and multiplayer we will see all sorts of emergent game play.
These individual gameplay building blocks, not all that remarkable in themselves, become amazing when they interact in a vast galaxy. Adding planetary surfaces, and interiors to that increases that effect massively.
So it's not about just sticking a FPS on to Elite and all bunny hopping around looking for head shots. It's about opening up more of the Elite universe. I imagine a large chunk of it being none-combat.
Examples I can imagine:
Passenger missions - now instead of just ferrying passengers to star ports, you take high end passengers in to orbit in your liner and then ferry them down to beauty spots on the planet's surface in a shuttle. Or on a pleasure flight through canyons.
Photography - media outlets will pay you to take photos of never before seen worlds. Shots from space, in atmosphere and hard to reach locations on the surface might all pay differently.
Mining - now you can scan for hard to detect ore on planets, deal with hostile atmospheric conditions, strong gravity, electrical interference and the like, and bring back geological samples.
History - board ancient spaceships adrift in space and bring back artefacts. Explore the ruins of abandoned colonies on remote moons.
Science - Bring back botanical samples unique to a particular habitat. Collect evidence that intelligent life had visited a particular planet. Capture rare animals for a zoo.
Spying - Frontier style photography missions. Or getting on the ground and scanning a particular area or facility. Stealing research items, etc.
All of these might have their own progression paths, along with special equipment load outs. No longer just limited to a couple of militaries, you might now gain status with a scientific research organisation. Which would then open up more ambitious missions, and give you access to specialist equipment. Or perhaps by discovering many unknown beauty spots, major tourists companies would give you more lucrative missions, even to take notable NPCs to see a unique alien sunrise.
Naturally there would be criminal possibilities with all of those. Smugglers may want to loot those historical artefacts. Poachers may want you to hunt big game for them. Pirates may want you to stop outsiders from taking an interest in the systems they use as a base, etc.
It's where all of these things interact that the magic will happen. You're flying leisure cruises and someone approaches you to steal research from a notable passenger's cabin while they're down on the surface. Or you're collecting plants for research when your scans detect evidence of alien artefacts on the planet. But now you fly off with that evidence and you're intercepted by ships who want to keep it a secret...
The possibilities are endless. It just magnifies the scale and scope of this amazing universe.
Elite had this remarkable scale. Frontier took that to a whole new level with this vast realistic galaxy in which you could fly to a distant star, set down on an alien world and watch the suns rise. E : D can go a step further, being able to step outside your ship and walk in places no-one will ever step again. That sort of scale is awe-inspiring. That's Elite. Not just doing the same thing again but prettier. Creating something that is truly mind-boggling even to those who saw what came before.