Researching Hiit Interval Timers

I have been trying out a few HIIT workout timer apps lately but haven't really liked any of them. Either the interface is on point but the features are lacking or vice versa. I want to make something that is a joy to use. Something really thoughtful.

To start with, I'm going to browse HIIT timers on the Google Play store and capture all of the likes/dislikes mentioned in the feedback comments.

Likes

Paid for the premium version to be able to save more workouts.
Customizable voice notifications!
Really easy to build your own workout.
Clear, easy to use and does exactly what is promised.
Very easy to organise and has a clean design.
It would be nice to have an option to put a description/note for each exercise so that we'll remember how to do it.
Great customer service.
It would be awesome if it could run in a small window over other apps like YouTube or at least if it could continue to run in the background.
I'd like to be able to make adjustments in the times after they have been set.
What would be nice is an option for us to put small thumbnail size images next to each exercise.
I'd love it if the last tone on the final interval was extra long so I'd know without looking that I'm finished.
Will be very nice if there is a preview of the next exercise so I can get well prepared during rest time.
Completely customizable. i was able to set up my own workout with varying exercises, times and rests.
Downloaded for its voice notification for each set.
Personally I liked the boxing bell sound because it makes me feel like I finished a round and won!
Updates are always useful and add a good feature like being able to save your timers.

Improvement needs

It won't let me open my keyboard to write in a name for a set/exercise. It pops up then closes right away.
Will not let you save two workouts until you pay for premium. Should at least let you save two.
Annoying ads on close/open.
It quits the workout if you close out the app.
I couldn't get it to work in the background consistently while I was doing emails and other computer tasks (pomodoro).
Big problem is it tends to crash or stop timers when you lock the screen. Really annoying problem, especially if you go for a run and just want to listen to the notifications of when to slow down or speed up.
The arrange feature works erratically so one is left trying to order the exercises.
Would have rated it higher if it stopped spamming about its Facebook group all the time, despite paying for the premium version.
Wish it counted down rather than up and had sound.
Just wish if there was an additional voice prompt which would indicate whether its a work period, rest period, prep time or cooling period.
I'd like the pause/play buttons to be larger. They're relatively small in the bottom corner and could be enlarged.
Has no integration with Google Fit or any other fitness tracker.
App works but the new app icon with yellow border is too hideous to keep installed on my phone. I hate looking at it.
Wish you could lable each of th erounds and have the option to set each round a different time.
Wish I could change the countdown time.
I can't define how many intervals I want to repeat.
I would appreciate if you add the pause option.
It should be configurable with how many seconds left it would start counting outloud.
The UI could be improved, especially for tablet use.
Looking for an app that allows varing set lengths.
When headphones are connected via blue tooth, it will not play the loud beeps through the headphones but instead through the phone which makes it unusable at the gym.
I want to be able to change the background colors.
Could have an option to extend the intro countdown. 5 secs is too short.
To add a vibration would be another nice option.
Can you add a countdown timer option that gradually gets louder?
Awkward portrait mode display (portrait and landscape)

Users

Runners who want to listen when to speed up or slow down.
HIIT exercisers who want to customise their own workouts.
Self-starters who want to use the interval timer as a pomodoro clock (running in the background while they do other tasks).
People who are recommended exercises from the hostpital, to make it easier to keep track of number of sets and reps.
Gym goers who want to work out using headphones so as to not disturb other people.
Tablet users who would appreciate a responsive interface.

Thoughts on Feedback points

There is a lot of excellent feedback points here. Most of these things are hard to anticipate. I would never have come up with half of these by myself. Interestingly, many of these feedback points were repeated across the top 5 apps on the play store, so it seems like the app developers didn't go through the feedback before hand like this.

It is also easy to get overloaded with all of the 'nice to haves' in the list above. So when I build this, I'll start off trying to make it as simple as possible. I'll really need to practice encapsulation and extendability, because I know there's always going to be good feedback and change/improvement suggestions that I'll need to be prepared for.

This was fun!

Simplest version

Next is to come up with the requirements for the simplest version of this app that I can come up with that has an exceptional user interface and generally just feels amazing to use, even if it doesn't have all the 'nice-to-have' features yet.

For my own personal use, I want to be able to create a workout that has any number of sets that each consist of a workout time and a rest time. The sets can be duplicated, or created with varying times. Each set should have a name. The workout should also have a name. The workout should also have a customisable warmup (or intro time) before the timers start. A countdown on the screen for each of the timers. A beeper which counts down from 5 seconds and below. A voice which says '[time] warmup', '[time] workout' and '[time] rest'

When I make this, I'll have to break it down into tiny simple steps, because there is a lot going on there in my ideal mvp that actually does the bare minimum I'd appreciate.

Some ideas for things that would be nice to have: Being able to see what's coming next.