I made a post somewhat like this one a while ago, but whatever I didn't talk about all my ideas in that one and everyone probably forgot anyway. So whatever:
Make it actually powered: It would be pretty cool and actually interesting if it ran on power cells or microfusion cells like the lore said it did, and depending on the upgrades or type of fuel source you put in like fission batteries or something it could last longer and work harder.
Immunity to small arms and basic melee weapons: They have durability on it now so it won't be that OP, but it would be cool if it had the strength of power armor in the first 2 games, where practically any non-energy weapon or mutated beast like deathclaw's and such couldn't even deal damage to you. I also really thought it stupid that you could get plinked to death with raider pistols and baseball bats when they act like in the lore they could take over entire armed chinese towns solo with the basic t-45 armor.
Fear factor: I always thought it funny that poorly armed and cowardly raiders and such would laugh at the challenge of facing a giant man in Power armor with miniguns, missle launchers, and fat mans. I think they should, ya know, run away from the monster in a mobile tank. I guess you could potentially argue that their so hopped up on drugs that it makes it hard for them to really grasp the situation, but they still run away when you start beating them in combat anyway.
Mods: Obviously this is confirmed to be in the game so yeah. But still one of my things.
Overall badassness: I always never really felt that powerful with it in FO3, it just felt like glorified combat armor, especially when basic super mutants and raiders could still easily kill me for some reason. They made it a little better in NV but still, I wish it still had a lot more awesomeness to it.
These are just my ideas, what do you guys think about it yourselves?