I've had a lot of time to think about this with some buddies.
I once wrote a long rant on this, but anyway... Hold on to your butts.
Firstly, I'd like if they cut down the scale a bit. Currently constraction figures are rather expensive compared to what they used to be. Sure, they also have more parts, but I think the larger size and price reduce their collectability. If they went back to 10-15 buck range, I think that'd be good. Something in the complexity a bit above the elemental creatures and protectors.
Second, some physiological variety. Repeating similar builds gets boring. Although uniters had different upper torsos, in the end they all looked mostly the same in their build and proportions.
...Which brings me to third: Break up some of the larger pieces. Make hips and shoulders separate again, so we can play on proportions and body style. Today the large chest pieces are rather ridiculous and difficult to use, so maybe divide those into separate chest and stomach armor.
Fourth, more technic in the builds to give them more complexity.
Fifth, masks packs for color variety. Also bring back axle-connected masks or some other more compatible method than the hot mess of G2.
Sixth, less CCBS and more technic-based builds. I really liked the second year of G2, but I think it could still be taken up a notch.
Seventh, smarter story and worldbuilding. Give us back the art in instruction booklets and tidbits of lore to raise the curiosity. If you make a TV show, give us a full season please with proper character development.
...Those are probably my main points, though I'd have plenty more to talk about.