For example 1 minute planks. A timer count down would help so you know when you can drop the plank.

For example: I had row intervals. Something like 10 min row with 30 seconds max pace followed by 1 min moderate pace. Another example would be something like a superset where there are multiple timed excercises meant to be performed with out a rest between (perhaps 60 second farmers carries into 60 seconds of abs...etc).

Any exercise that is bi-lateral, if it is 1min single leg deadlifts allow for a 30sec split or 1min per leg

My trainer programs amraps or to hold for certain amount of time. This and the Apple Watch possibility would make a huge difference!

Adding tabata timer and interval timer as appropriate.

Yes!!!!! Was just thinking that during my workout today. Pain to have to switch to my timer. Slows me down a bit from a flowing workout.

This.

If I could vote for this multiple times I would. I had to find a different app that would allow me to input a custom interval workout.

This would be awesome. It is an inconvenience to have to stop working out to check a timer, once I've gotten a good flow and my heart rate up. Even worse, the timer that is on there now only goes for a minute. Some of the exercises input by my trainer are to be done for less or more time.

Timer. Improve timer to accommodate timed circuits. I hate resetting timer for every set. BIG PAIN! Make bell louder and longer.
Eg. I do (5 diff exercises, 30 sec each, then 2 mins on bike) x 5!