Tesla has had a long term contract to buy 18650 cells from Panasonic Japan that won't be fulfilled until 2019 or so given current production volumes. Tesla can renegotiate the contract, but all 2170 production needs to be dedicated towards Model 3 and stationary storage for now. Once the GigaFactory is capable of producing more batteries than necessary for those needs will they switch for the Model S and X. That will be a while.
The exterior of the Model S is a classic design that is not out of date compared of other large sedans out there. At some point they may redesign the roofline to give a little better headroom, especially in the back, but that won't be right away. All the engineering resources for that are dedicated to higher priority projects right now. With any exterior redesign, they need to be careful to maintain the aerodynamic profile. The Model S is one of the most aerodynamic cars on the road and it should stay that way.
I believe the battery pack options will change before the Model 3 introduction, but not in the way the OP thinks. The large pack will remain the 100 KWH pack for at least a year, probably closer to 2, but the smaller pack is going to become the new 85 or 90. It will actually be around 88 KWH if my calculations are correct. That's what you get with 14X of the same module used in the 100 KWH pack. Tesla wants to only use one module for the Model S and X line which is what they had with the 75 and 90, the only difference was one was 14 modules and the other 16. Making a new 85 or 90 with a small pack would not only be good advertising, but it would also be economizing at the factory.
The interior will probably get a bit of a refresh in the next year or two, but it will probably be after the Model 3 goes on sale. There will be some people who will decide to buy a Model S instead of a Model 3 after the production version is revealed because the Model 3 won't be what they thought it would be, and there will be people so far back in the Model 3 queue they will opt for a Model S instead. (Among those who can afford the bump in price.)
Tesla might also do a price reduction on the Model S that will entice people on the fence. The car has a massive profit margin now. It's needed to fuel all their future projects, but they could cut the price and still make a tidy profit.
I don't expect any major interior changes and the possible roof-line change until the Model 3 is out there and is getting produced in large numbers. Most of their manufacturing engineers are needed to get the Model 3 lines going and they don't have the resources to spare for retooling the Model S line right now. There will likely be some tweaks this year though. The battery change from 75->85 is most likely IMO. Other changes might include the addition of a HUD so it can be tested out in the field before getting incorporated into the Model 3 as an option.
There may be some electronics improvements here and there, but again, nothing dramatic. I strongly doubt there will be any major changes to the AP hardware until the software with the existing hardware is working as well as AP1 or better.
