Well it can't be an evolution of the current car as there are significant changes to the tub. Lower front bulk head, wider side pods, and new rear mounting points for the engine and gearbox and ERS kit. Then the nose is completely different, and the front wings. The rear wing is different too. The beam wing is gone. And the exhaust regulations are completely changed meainging the side pod rears are all new, and the floor is different due to the lack of EBD possibility. The diffuser is then redesigned to account for this.
Given all that, I can't actually think of anything the teams could carry over from the current car? Even the design phylosophy is no longer relevant for RBR as they won't be able to mould the air around the sodepods to meet the exhaust and seal off the diffuser.
Every major component is affected by the new rules.
But, I'm with you on RBR being quick. You don't loose this kind of momentum easily. The only real potential spanner in the works for them is the engine. There is a very real risk we see a 2, or even 3 tier F1 next year solely down to the relative engine performance.