Obviously, it's difficult for us - Reason users, not developers - to tell what's easy to implement, but I guess we can clearly assess that e.g. adding a keyboard shortcut for already existing feature is much easier than folder tracks or MPE. So, let's try to compile a list of such features for Reason Studios' consideration!
I'll start with my own and add stuff proposed by others as we go, so that the top post contains "everything".
1) Keyboard shortcut to make selected MIDI note(s) longer or shorter. It can already be done with a mouse, but it's "annoying" that you first have to click the note for handles to appear and only then drag them, so using shortcuts would be much faster, I think.
2) Have MIDI notes make sound when moved vertically with keyboard. This is already the case when dragging the notes with mouse cursor, but for some reason it doesn't work when using keyboard.
3) Ability to mute or solo currently selected track (when you're focused in Sequencer) or channel (when in Rack or Mixer), by pressing M or S respectively (plus perhaps Shift+M/S to unmute/unsolo everything). This helps to quickly isolate or exclude what you're currently working on, without having to often scroll around to find a tiny buttons to click, i.e. you might be editing MIDI of a track that's off screen or a device sitting on a channels that's off screen, too.
4) Automatically keep tracks belonging together ...together. I often build long / complex chains of devices on my tracks and use a lot of automation. The problem is when I'll add automation to an FX / insert FX, Reason will randomly place a new track and new automation somewhere random in the Sequencer and always with different color. At minimum, Reason should put it ALWAYS below the audio/instrument track and apply the same color, so that visually things are kept together.
5) The "Search all locations" option in Browser should be the default (or at least saved with projects & templates). This one's self explanatory - it's unnecessary friction to having to switch to the relevant cathegory, to find what you're looking for. It makes sense if search yields too much "random" results - instruments, FX, players, presets, samples - but that's rarely the case at least in my experience.
Please, post your ideas!

6) Markers, i.e. visual indicators marking specific arrangement sections, like into, verse, chorus, bridge, etc. + set of keyboard shortcuts to move between them for quick project navigation, like you can now quickly jump to song's start & end and loop's start & end.