So here is an idea of mine for the next expansion, which would hopefully revitalise the old world and simultaneously provide a lot of high level content. The idea assumes that we'll be getting a legion expansion next. So essentially the idea is a zone called the Burning Front, the centre of our operations against the Burning Legion within the twisting nether. Imagine the Isle of Quel'danis turned up to eleven combined with the Ahn Qiraj war effort event. Players would complete quests in the zone to build it up into an enormous military base, with the Frontier's story being continued in each patch offering new quest lines and scenarios. This wouldn't be phased but like Quel'danis would appear for everyone, giving a sense of progression for each realm. It would also be available to people of almost all levels. At level 10 upwards you can do a questline introducing you to the Burning Front, and from then on could contribute to it whenever you wanted. The questlines would take place around Azeroth, so say if the base needed lumber, you'd head over to Eastvale Logging camp and have to fend off a bandit attack to collect the logging shipment. Or say you needed some mana crystals. Well just head over to Silvermoon and do some backalley deals for those crystals etc. etc. phasing would ensure the mobs were the appropriate level.
So that's it really. A massive symbol of the realm's progress as a community, taking players into the old world and provide a cohesive, meaningful story.