I agree that Chaos War Engines would be awesome, and was thinking only yesterday that some Nurgley Towers and Catapults would be awesome.
However, I do really like frogbear's suggestion of a Chaos sprue/pack that has an assortment of motifs and turrets and daemonic faces to make or convert our own stuff with. It would also alleviate the issue of always keeping an eye on what FW are doing and then following a step behind. It would also free up people on budgets to get the vehicles they need for new lists more easily without needing a mortgage to shop on Ebay with.
Otherwise, I think that the Titan (and Chaos Titan) weapons systems could do with a rethink and remake. The old Titans are fine, easy to come by and not too silly money-wise. But the heads, weapons, tails and accessories... Not so much. Plus the old designs are still good, and it is mostly the details like the weapons and heads that need an update to 'modernise' the older models. Carl Woodrow's
20th Anniversary Warlord Titan is a great example of this.
Necrons could do with C'Tan and War Engines. I know C'Tan would be a low-return thing though as most folks only want one of each, but still.
A new Imperial Leviathan, Capitol Imperialis and Basilica Imperialis would be very welcome too!
Finally, some stuff like Squats could do with some new additional units, and Dark Eldar could do with most things. But I always got the impression that these were more 'fringe' armies compared to Tyranids for example, and could wait either until GW redesigns them or people come up with some good concept art. And the 'Nids need love from the ground up - we even know what they look like nowadays, but aside from some tempting previews I've not seen anything like a proper set of model proxies for them.
