I love zwift and have also used tried Trainerroad. I think it would be useful if you had the option to minimise the screen and follow a workout(erg mode) but not have to cycle around the island. Whilst in 9 time out of 10 I love the island, sometimes I would like to watch a film on netflix etc and just have the basic metric at the bottom on the screen. This is a feature on trainer road and turbotraining.co.uk. If this was incorporated it would negate the need to even consider TR (also once the workout section and plans are improved).
The other useful item would be the selection of workouts on iOS. On my iphone its really difficult to select a workout (because the selection screen is so small). It would be good if when you choose the workout dropdown (in iOS) if the selection and drop downs become full screen automatically. This would improve the user experience massively IMHO.