This applies to HR devices, but I am mostly concerned with those that display Power via ANT+ (I have an Edge 705 and Forrunner 310xt).
I know you can see zones for power/HR (not sure about pace though?), but often when training off plans they use a percentage of functional power to dictate the prescribed intensity. Since zones are in not regular intervals of percentage, or the same across power, HR, pace for running and biking, it is very hard to remember the % Power range Zone 3 is, and it would really be nice to have a data field that just says "105% W" or "88%HR". Not of a MAX HR, but your threshold. You would enter this number probably under your user profile and could be synced wtih GC. Your critical/functional/lactate (whatever you want to call it) threshold is most appropriate, but people could have the freedom to put whatever they want and update it as you progress and improve (or not).
This seems really simple and would be AWESOME to have!