It would be really swell if, on a simple instrument track, a person could set up automation lanes for MIDI controllers, not just audio parameters.
I understand the current workaround is to either split the simple track into separate Audio & MIDI tracks and set up controller automation lanes on the MIDI track, OR to just start with separate tracks to begin with. But since the Simple Instrument Track exists (I would assume) to conserve real estate and make working with soft synths more efficient (thereby bypassing the need to set up MIDI/VSTi routing), locking you out of MIDI controller automation is definitely less efficient.
Since the Simple Track is in effect a unification of an MIDI track and an Audio track with all routing/connections being done behind the scenes, it would stand to reason that all of that behind-the-scenes information could still be accessed fairly easily with the interface. If making both Audio & MIDI automation accessible within a single automation lane would be a hassle (since it's technically treating it as an audio track at this point), perhaps adding another button/drop-down for MIDI automation would be feasible.