I believe Tesla has been focused on high margins for the S/X to fund development of the Model 3. Since there is little to no competition, introducing premium luxury features has been lower on the priority list, as those features add cost and have not been necessary to drive sales. Also, adding extra options increases weight and therefore decreases range.
BUT!! Now with the impending introduction of the 2170 cells, weight and cost will go down significantly, and Tesla will be able to afford to add extra premium amenities without drastically lowering margins or reducing range. As Porsche/MB/BMW/etc introduce and refine their elecric vehicles, there will be some competition in the market, and Tesla will be forced to match its competition.
I think we will see:
1. HUD/new steering controls - looks like it is coming to the Model 3, so I imagine it will come to the S as well (sooner rather than later)
2. Improvement in the center display/MCU - if full self driving is coming with AP 2.0, the driver is going to need something to do. Having robust media/internet capabilities and perhaps some sort of innovative phone integration will allow the driver to perform useful tasks while AP is working but also still keep attention directed forward in case the driver needs to take over. I personally think the goal of having full self driving in place next year is an ambitious one, but if they truly want to introduce this functionality by the end of 2017, they will need to keep the driver entertained.
3, Better rear seats -- I think this is a big one. Not really for the US market but for the Chinese market. MB/BMW/etc have special long wheel base models for the Chinese market with executive seating in the rear, and apparently, this is a big deal. I could see Tesla reintroducing the executive rear seat option and adding rear creature comforts (4 zone climate control, rear seat entertainment, etc). This may not make it to the 2017 year though.
And to jump the gun on 2018 -- I think we may be in for a substantial redesign of the interior (and possibly exterior). Major design changes could be in the pipeline to coincide with the introduction of other viable premium electric sedans. A substantive redesign would allow for refinement of AP, changes in major interior components like the center display, audio system, etc.
Exciting times ahead for Tesla! Hopefully, someone will find this thread on 12/31/2017 and we can look back and laugh on how right/wrong we all were!