My clients have been using Everfit for the past few months, and while Trainerize has a much better workflow and options, clients feel like the app is a step-down compared to others in the industry, simply because the UI doesn't seem as 'slick'.
Without changing the skeleton/framework of the app, you could modernize the UI to match current market trends, using mini-animations, colour schemes and other UI modalities.
As they say - packaging is everything, and we want our clients to feel proud of the app they use.

Have the client window automatically refresh when I choose to open a different client's profile. Right now, you open a client profile and a separate window opens for them. It would make workflow easier to be able to keep that window open and it automatically refreshes once I'm done with one client and click open on another client in the main window. It feels clunky to have all these windows open and having to constantly click on "Refresh and view CLIENT's account"

Add webhook to notify when someone cancels their auto-renew

This would be an awesome feature so we could see our long term business growth. Also be another selling point for people to join up with Trainerize

If clients can "tick box" or "agree" to a wavier, T&C for programs in the purchasing stages this would simplify the process and that we don't have to chase clients for them. It could also mean that dont get access to a program until they have done this / signed the wavier etc. This would reduce our risk of a client automatically starting a program if a waiver has not be signed.

It would be convenient to allow admins/trainers to be able to download the video they've uploaded through the trainerize app. I know I can just record my own and upload it. Its nice to be able to select the exercise and record through the app, however if I want to use that video for something else, downloading it straight from the app would be great. For example if I delete it from my device or all other storages, its still there.