In many ways... Wow brought in all characters and is killing them off. Illidan, Arthas (upcoming expansion) were main characters that have been killed. Kael, lady Vash (dont remeber how to spell her name) were killed. If they made a Wc4 they would need to make a complete new story line. Or base it in the future on the same lands (maybe the legion will come back or orcs will start to rape again).

All in all Wow was like a gaint money making peace off crap that fell on WC series. The chances that a WC4 will come out and relate to any of the previous characters is slim because it wont match up to WoW.

In 10 years if they make a Wc4 it will relate very little to Wc3 (besides maybe the land) just like Diablo 3 (which is set in the future) and Doom 3 expansion.