Though we don't have any news about allowing third-party interface addons, we'd definitely be interested to hear what functionality you feel is important to add to the current UI. Others have offered a few suggestions (thanks!), and it's helpful to us if you describe what you think is missing and let us know what we can do to improve the UI!
The Big Three (As far as I'm concerned..)
1. Buff/ Debuff sizing and organization. Even as small as something like removing them from the self/target frames so we can resize them alone.
2. Mouse Over Healing, I would love to see guard go on this list too.
3. Some form of threat indicator. (This is MUCH less of a problem since 1.3.) Even something like healthbars changing colors, I would say we don't need exact numbers. (For Tanking Green=Have threat, Yellow=About to Lose Threat, Red=Lost Threat)