The Chaos and Imperial cruisers are pretty cool models with good details. I think the metal ships are pretty decent too, as well as all of the Forgeworld stuff (resin). There was not a bad or ugly faction theme/line in the bunch. If/when they do it, I think keeping with the semi-modular theme that they already had with the Chaos and Imperial cruisers would be the way to go. I really like what Hawk has done with their Dropfleet Commander line, and it really allows you to build what most appeals to you visually, with multiple choices on what variant of a basic hull to create (just make sure your opponent knows that your destroyer looking thing is actually a carrier). As Spartan games is toying with, modular might (and should) the future.
Selfishly, I say no redesigns, as I don't need another game to spend cash on and have too much invested in the old stuff. Just expand the lines. Unselfishly, I am sure they could update stuff to look even better. Ruck, there are some kit-bashed wicked-cool Dark Elf battleships out there.
Its been SO long since I actually played, I don't remember what I might not have liked. Necron were strong as are the Eldar, and we may have had some house rules to nerf them a bit, if I recall though. And if I remember correctly, yes, whole factions activated simultaneously. Not realistic in the least and a few die rolls could decide the game in the top of the first inning! Squadron activation, please. And few, if any, new rules. Go more Full Thrust than Star Fleet Battles.