It'll be interesting to see what GW does with the empire in the next wave. Currently, empire is the one army that is almost entirely plastic.
*All* of our troop choices are available in plastic. Yes, the knights are mediocre. I could see them replacing the knights, the milita, and perhaps the cannon/mortar kit due to age, but everything else is new.
We have new plastic kits for the fighty heroes and wizards, so all we really need character-wise is some mount options (griffon, pegasus, dragon), a WP kit (perhaps with a war altar), and an engineer kit.
Most other armies are in *much* worse shape, model-wise.
GW did such a thorough job of updating the models for the empire that there's almost nothing left to be done for the next edition (barring new units, of course!).
What GW really needs to do is tighten up the playtesting and rules development side of the game. Once an unbalanced armybook appears, it skews the whole game until it is replaced.
And the whole thing with releasing new army books over a multi-year period is asinine. Just as the last armies finally get thier new armybook, there's a new edition which completely changes things.
I'd much prefer to see a core armybook that contained the basic rules for *all* armies. Expecting the players to shell out $300 per edition, just to get all the armybooks so that you actually know the rules is ridiculous (and increases the likelyhood of piracy, IMOHO).
This approach would put all of the army rules in one handy package. The rules would be developed all at once so that balance could be more easily achieved, and you'd only need one reference book instead of 14.
The armybooks could then be released, but instead of laying out the army rules, the army book could focus on background, strategy and tactics, and the modeling aspects of the hobby... stuff you don't strictly need to play the game, but things which enrich the game for the player of that army.
So at the beginning of the new edition, ALL of the rules would already be in place. Nobody would have to wait 4-5 years to get a playable book for thier army, only to find that their army was once again screwed when the next edition came along a few months later.
Miniatures would be released periodically over the course of the new edition. Over the next few years, all armies will get a more-or-less complete makeover. New units could be added on an experimental basis during the cycle... not official for tourney play, but simply to test them out and get the rules balanced. At the beginning of the next edition they'd be offically added to the roster. New armies could be done the same way.
That's what I'd like to see.