987 Boxster production finished back in June so how can they still show the 987 at LA? 997 cabrio production is still going and my dealer says he is ordering those models now for February delivery. The 987 Cayman is staying in production until March so it certainly won't be that model and it would be crazy for Porsche to show a new 550 model before the launch of the 981 Boxster as the 550 will be a cheaper car to buy and could steal 981 sales if shown first.
To add further intrigue Autocar were reporting that Porsche may delay the introduction of the sub 918 V8 car as they want to develop the platform for it and not use the second generation R8/Gallardo platform which they have had no input into. I would take that with a pinch of salt though as Autocar said that could mean delaying the introduction to 2020.
The only other model that could be conceivably displayed to test the market reaction is possibly the Cajun as an early styling exercise? Now they have access to the Q5 a reskin of that model should be possible for a show display car.