Removing Auto OC will probably just make everyone play better since they will actually target the right units with OC and get some multi-kills. Plus manual OC has a quicker cooldown. There's almost no circumstance where I ever use auto OC, and think it is a mistake for other people to be using it. I'd be happy to see it removed though.
I don't think halving ACU gun damage would affect T2 land usage much, it already takes like 10 seconds for a gun ACU to kill a T2 unit without overcharge. It would drastically reduce the ACU's effectiveness against T1 spam, which is probably not desirable. Most likely I would hate this change since I like powerful, offensive ACUs, and I'm afraid of what 1v1 T1 spam gameplay might look like. It might make auroras OP too. I'm not opposed to trying it though.
I don't like making T2 units in general because the game is very unlikely to end on the T2 stage, especially so in teamgames, so all the T2 units will be obsolete. T3 units never become obsolete, so it is never bad to make them. By the time you invest enough in a T2 army that can easily start crushing T1 spam, you could have made a T3 HQ and just countered the T1 spam with your own T1 spam. Unless there's some coordinated T2 push across the entire team, going T3 quickly seems like the much safer play.
Way back in 3655 T2 HQ and support facs received big nerfs because apparently T2 rush was too common. The HQ nerfs have since been slightly rolled back. Since then the range bots have been nerfed, T1 bombers made cheaper, Aeon T2 made decent, and other T2 rebalanced, I think it would probably be okay to bring T2 HQ costs back down a bit. I think the T2 movement speed changes were good, and I've always felt a nerf to T1 movement speed would be nice as well. Slowing down the somewhat frantic pace of the T1 stage and giving you more time to react and not get shift-g'd. These changes would let you have T2 units a bit earlier and make it harder to hold them off with just T1 spam, meaning you would need to actually participate in the T2 stage.