Problem is, it’s not just Silvermoon that’d need to be revamped in order to enable flying — it’d be a good chunk of southern Eversong and Ghostlands as well.
This is because Eversong+Ghostlands aren’t on the same map as Eastern Kingdoms, but instead is a chunk of land on the Outland map floating out just beyond maximum view distance to the east of Hellfire Peninsula (if you turn your camera just right while standing on the edge of HFP, you can see its outline).
Because it’s a floating chunk, the map artists never gave much thought to making the map physically compatible with the Eastern Kingdoms map. As a result, the lower southeast quadrant of Ghostlands (Deatholme) actually occupies the same physical space as Stratholme in Eastern Plaguelands. More details including pictures can be found here.
So enabling flying in that area would be a significant effort. I’d love to see it happen, but it’s not likely.