To help illustrate things, I decided to list the current instruments (w/o the retired models):
Prophet-6 Keyboard
Prophet-6 Desktop
Prophet 12 Keyboard
Prophet 12 Desktop
Pro 2
Tempest
Prophet '08 Keyboard
Prophet '08 Desktop
Mopho x4
DSM01 Curtis Filter Module
DSM02 Character Module
Those are 5 synth engines in the higher price range (Prophet 12, Prophet-6, Pro 2, Prophet '08, and Tempest), one clearly affordable 4-voice polysynth in the medium price range, and two Eurorack modules.
Taking into account that the prices of the Prophet '08 Keyboard and Module, the Prophet 12 Module, and the Mopho x4 have been lowered recently, one could be tempted to assume that those will be next on the retirement list - minus the Prophet 12 Module. At this point, that's of course pure speculation. Nevertheless, the Prophet '08's DCO engine (up to the Mopho x4) is the second oldest one DSI came up with, which seems to make it logical that this line of instruments is going to be next in line.
This allows for two conclusions: First of all, it's certain that DSI has now a gap in terms of low priced gear. Secondly, DSI might soon be left with only hybrid instruments, plus the Tempest. From that point of view it seems likely that further analog engines will be exclusively developed under the banner Sequential.
Either way, since both brands - DSI and Sequential - have to be kept organized in tandem, I wouldn't expect an immediate replacement for each of the five retired instruments, simply because of logistical restraints.
What I could imagine is also a monosynth based on the Prophet-6. Other options include a couple of more Eurorack modules. The Pro 2's oscillators come to my mind (I'm mentioning these exclusively because of the 13 Superwaves) as well as maybe some kind of sequencer module.
Other than that, I could definitely imagine entry level synths that are based on the Prophet-12's/Pro 2's architecture, but I have no clue what these could look like. Maybe, just maybe, a desktop version of the Pro 2 might still happen one day although I couldn't see the benefits if it's not approximately the same size as the Prophet-6 Desktop (i.e. Pro 2 minus the keyboard). Otherwise its whole appeal would be lost, and who would like to menu dive to enter the sequencer mode for example (that includes the option to record the automation of certain parameters on the fly for which you need a couple of knobs at hand instantly)?
All in all, though, these are all ideas based on already existing engines. Mainly because I can't imagine DSI/Sequential to release yet another flagship synth by early spring. If they would do so, it had to be something entirely different from the Prophet 12 and the Prophet-6. Given the developements in the Eurorack scene, the options for a new engine are countless. It would have to make sense as a stand alone synth though and be worth more than the sum of its parts. But I'm also confident that DSI/Sequential is able to surprise us with something we might not even expect them to do.