Marler's only 30. If his head's in the right place he can make the next RWC. Genge would have to be the next cab off the rank - he doesn't always get his scrummaging right, but when he does he's very good and I love the way he tears about the place, providing some of the aggression we sometimes lack.
Lock wise, Launch is in form, experienced and is just the heavy duty kind of lock that we need, full stop, but esp if being by paired with a more athletic type - maybe not quite as stellar in the line out, but more than capable and has really seemed to grow into his leadership role at Pests. Isiekwe's even bigger and is great in the line out with a solid all round game that he can add frills to over time.
Hill's a more athletic type, but I really think you need at least one heavyweight bruiser in the engine room. I doubt Skelton would ever have won any plyometric tests, but just look at how Sarries got him fit and then harnessed his raw power to the benefit of the team. He was outstanding and probably at a higher level than when he was playing internationally.
Thought Attwood should have won far more caps than he did.