The Switch is not "tapped out" in terms of OS features, but it also probably isn't the development focus of their firmware team anymore. Drake is clearly taking priority right now, and probably will continue to do so after it releases. Since they appear to be developing the changes within the main Switch OS repo this time (as opposed to a fork like they did last time with 3DS) we can probably expect Switch will continue to benefit from any new features that get added and aren't specific to the new hardware for some reason, up until the point the system goes fully EoL. I think at least a minor visual refresh a la 3DS 9.0 is probably in order.
In terms of what to look for in terms of predicting Drake, the more obvious things would be features or modules that are either present, but unused, or not present, but their existence is implied. 15 and 16 (so far) have been accumulating a lot of little things like that. The one other thing that might (emphasis on might), show up is something to distinguish Drake games from Switch ones, but there are ways to implement that that would be very implicit on the Switch side. From what we've seen so far, most explicit Drake stuff appears to be ifdefed off.