I was initially cynnical about how well starcraft 2 would turn out, but they demonstrated they were able to churn out a good rts still.. as far as WC4 goes, I'd absolutely love to see that happen to add some continuation, and possibly some "closure" to the story side of it (setting aside the brilliant multiplayer) as alots happenned in the years since the events of wc3
Traditional competition in the rts genre (ea/cnc) has gone down the toilet, and despite sc2's success I definatly believe there is room out there for a new wc rts.. after all, RTS is not a "1 size fits all", people have different preferences in the style/environment of it.. which is a big part why blizz enjoyed a double success with starcraft alongside the original wc games.
Maintain the quality, theres a couple things which could be slightly "modernized"
Stick to what works, if its not broke don't fix it, and you are good to go!
Hope it happens