If I was taking a wild guess at what will happen...
There will be both a hatch and sedan Si model. With the coupe being gone, and the hatch taking it over in sales, it makes sense to offer both body styles in Si form. Second bit... the EXL trim is now gone, that leaves a sizeable pricing gap in the lineup between EX and Touring. I believe the new Si models will slot into this range and feature a mixture of EX and Touring level equipment, mostly EX level, but perhaps getting the fully digital instrument cluster, as well as the Bose audio, two features I feel would mesh best with the Si, while keeping the smaller 7" infotainment from the EX level. Obviously interior would be cloth as well.
Second prediction, first ever Si with an auto OPTION. Both models will remain standard with manuals, but will offer either a 8- speed DCT (sourced from the acura ILX) or 10-speed auto option (from Accord). Not only would this vastly expand the sales potential of the Si to a wider audience, but would also help the civic lineup mirror the accord lineup more closely. The accord sport is offered in both base engine and upgraded engine variants. We can imagine that the Si is essentially the upgraded version of the base sport trim for the Civics.
If I was going to try and guess at other upgrades the Si might get... possibly a power bump to 220-230hp. It's already been proven that the 1.5T can handle more power thanks to Hondata and Ktuner. With a sturdier transmission I don't think a 15-20hp bump is out of the question. Considering the Hyundai veloster turbo (the closest competition to the Si in terms of content and price) having a 0-60 time of 5.9-6.2 seconds depending on transmission choice, the civic si needs to drop a few tenths off it's current times to stay competitive.