WIth 5.0 no doubt seeing a lot of active work but hopefully not being fully locked in, this seems like a natural time to contribute ideas. Personally, I would look into making healer dps more interesting (smartheals on damage like at least used to exist in WoW is one of the simplest ways to make dealing dps feel more meaningful for healers off the top of my head as an example), do some serious UI reworking/add features for much more heavy customization, re-do roll actions (as part of this rescue needs to be made more useful and maybe some interesting options like non-party locked aoes, different shapes and types of aoes, useful cc, and strong but non-obvious utility like rescue), and, um, well I'll try to go into more details in a few specific examples below.

1. Show us the time left on buffs on allies (even just a clock type of darkening/lightening so we can estimate how long a hot or other buff has would be huge).
2. make it so the party window can be split into two movable light parties when in 8 man parties (with my personal set-up this would leave me with one window having tanks/healers and the other having dps).
3. Add a proc that makes the next healing spell instant and not use a gcd say at a 5-10% chance on every cast of the default filler dps spell for each healing class.
4. Make rescue not feel laggy and actually fit the obvious use cases of the spell in real combat.
5. Allow macros to be queue'd so I can create 'smart' macros for my heals allowing optional mouse-over and other niceties!