Include a function in which you can pre-plan supplement consumption as matched to workout phases. Would be handy to be able to keep track of supplements within the app

Show a live / real time calorie burn on the app screen during workouts ….

I think you should pursue development of a bluetooth accelerometer band that straps to the bar/wrist/ankle. These mirror based home workout devices are getting popular, but they aren't great for those of us needing squat racks and legit weights. But they do a good job of using computer vision to provide feedback on form, range of motion and tempo. I originally thought using the cell phone for real-time video analysis was a good idea, until I realized 80-90% of the benefit would come with an accelerometer with much less hassle. You would strap the device to the bar/wrist/ankle and do the warmup set focusing on perfect form and tempo as usual. Then on the work sets the accelerometer would feedback to the app whether someone is maintaining form, tempo, and ROM. Interestingly enough with regard to RPE, many studies more accurately determine the optimal rep limit by instructing the lifter to attempt to maintain steady tempo and then determining at what point the rep slows beyond a threshold (e.g. 20% slower). The lifter doesn't have to estimate how many more reps they could've done. They would just go until the most recent rep crosses the tempo threshold and stop. And it could easily tell me to watch my tempo, form, or ROM during the set. Almost everyone uses headphones during the workout anyway. Feedback like, "You cheated that rep a bit. Get deep enough." When my reps start to slow noticeably. "OK it looks like you've got at least one more in you." "That was a great set. Let's rest a minute." I've thought a lot about how I would accomplish this, but frankly I'm neck deep in developing a couple other things, so have no desire to get into workout devices. But happy to discuss further if you'd like.

An option to be able to pre-set rest timers by exercise within the settings page for each particular exercise would be awesome. This way, you can customize your particular workouts even more! 

I’m getting drop down notifications to say get ready to exercise when I’m on rest  days ….

Could the system swap out for incline or dumbbell press and add in flys?

It couldt be nice to be able to set de bar weight to zero.  In some exercice, like hacksqat there is no need ( at my gym ) to consider the bar weight