User Interface upgrade
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.
Rob Yontz commented
This would need a very specific scope / outcome defined in order for me to vote and support. I have seen vastly too many decent apps attempt something like this and become much less "professional" by going too "cartoony" and "gamey".