OK, a post for pure speculation! Nothing here should be taken too seriously, and feel free to make the discussion "just my opinion", since we don't have any actual information - so no need to remind us all about this last fact! :)
Let's begin ...
The expectation is that in a few months Garmin will release the x4x series of watches, which seems to happen on a ~2 year timetable (excluding the oddly positioned 225). These will be upgrades to the 230/235/630 running-focused watches, with whatever new goodies Garmin has in store (and probably some higher-end features already around that will trickle down).
Earlier this year we had the Fenix 5 and 935 released. These are Garmin's top-of-the-line multi-sport watches with all the bells and whistles. The timeline there also seems to be approximately a 2 year cycle (Fenix at least), meaning we can expect the next significant update in early-mid 2019.
So where does this leave the 735XT in terms of its natural upgrade pathway?
To me there doesn't seem to be anywhere Garmin can take the 735XT that doesn't already overlap with an existing (or expected) watch. Was the 735XT simply a transition product while they finalised the 935, and it will hence go the way of the 225? Or do you think there's something genuinely new/unique they can do with it that maps out its own space in Garmin's product line (a 745XT, perhaps).
What do you think?