Ten years from now, I will be surprised if "four door coupes" even exist. The idea made more sense when sedans were mostly of the traditional three-box variety - thus allowing "four door coupes" to stand out more dramatically, but the market has spoken and people clearly want more svelte, sexy sedans and they don't care if you call it a coupe, sedan, scooter, crossover or anything in between. The only way I can see "four door coupes" living on is if they become even more dramatic - something like the LF-LC's translation from concept to the production coupe LC 500 but with two extra doors - it would have to be that dramatic and almost shocking. The only one that I've seen that fits this mold lately is Mazda's Vision Coupe Concept:
Furthermore, with the market for passenger cars contracting, it will become even more difficult for automakers to justify offering two sedans at a similar size/pricepoint. Lexus ES and GS are great examples of this. I think we will see all sedans moving to a more shapely, fastback form... just like we have already seen with the LS. I believe ES and IS will follow the same philosophy when they are redesigned.
So that brings us to the RC. As Kevin has already pointed out, it's kind of hard to see what else could be done to help the current generation, especially considering the minuscule sales volume. Based on the released photos of the RC F and GS F 10th anniversary edition, we already know the car is getting the same 10.3" screen the refreshed IS received. Other than that, I obviously expect an updated front and rear end and hopefully some suspension tweaks. I assume they could rework the RC F's intake system to get the same 471hp the LC 500 has.
The thought that Lexus used the IS C's center tub because they ultimately planned to make a convertible makes a lot of sense, but weight is the RC's biggest enemy and it's all for no reason now. There's nothing they can do to remedy it until the next gen. Looking at LC 500 and LS 500, GA-L hasn't proven to be exactly be featherlight either...
If the RC lives to see a second generation, I have a feeling we'll see it more closely aligned to the IS than the current IS/ISC/GS Frankenstein car. Developing it alongside the IS and keeping the differences between them minimal would reduce manufacturing costs and also make it easier to engineer an IS F and RC F side by side. Seeing that the GS could be dropped from the lineup, this closer alignment with the IS makes a lot of sense to me.