Neutral Villagers... disagree. I think the combat behavior of villagers is fine now. I do think there is an argument for villagers to have guard dogs (tamed wolves, players can't claim them) who will assist in fighting off enemies, but I don't feel the villagers need to do any fighting.
Iron Golems... yes to being a little more forgiving of hits (especially if your reputation is high), no to being weaker
Trading... expound on this. I don't think we need a whole new money system, but I could see Emeralds being broken into Emerald Shards, so you could have more granular trades like 8 Wheat = 3 Emerald Shards. That also would be nice for RP servers that use Emeralds for currency since you could have a little bit more depth to currency systems. I do also think we need "treasure" villagers, which is to say rare villager variants that you have to "earn" rather than just finding in a village but who in exchange offer especially valuable/rare resources (i.e. a Cultist who sells Nether/End materials, an Oceanologist who says aquatic blocks like Coral, a Spelunker who sells cave blocks, or even a Zoologist who sells single mob eggs at high markups). These could replace the Wandering Trader, who I find to be kinda... pointless at the moment, and/or the Wandering Trader could be changed to just have randomly selected trades from villagers. In short, I agree that the whole trading system could be upgraded, but I'd like to see you flesh it out
Buildings... Sure, yeah. I'm fine with buildings now but as new materials come out, I'd like to see some of those incorporated. Also, village generation is still kinda meh, so I'd love to see that revamped again, too