I've been stalking this thread since its days on Terraria Online, and it's definitely something I'd like to see put in the game. However, there are are a few issues I've noticed that I think could be resolved one way or another without changing too much about this suggestion.
Firstly, it's the idea of having the Peacekeeper as a mechanical boss for the Queen Bee (Yes I do know that Peacekeeper is only referenced in this thread). This doesn't appeal much to me because I've always seen the Queen Bee as a sort of side-quest boss; no one is forcing you to fight her, similar to King Slime or Duke Fishron. Sure she's really easy to find and battle, but she isn't absolutely integral to game progression unless you are planning on being a summoner. Based on one of your other suggestions about adding a Grand Harpy boss to the floating islands, it only makes sense that the Queen Bee is one of those side-quest bosses since she's important to mostly only one type of class. For these reasons, I think that the Queen Bee should not have a mechanical version and only the Motherboard should be a new addition.
Secondly, kind of a small thing that's bugging me a bit is the whole soul of flight issue. Thinking about what I said above, without the Peacekeeper boss there's still an opening for a seventh core, and this could be a core to match the soul of flight. By doing this we keep the soul of -ight theme and and seven deadly sin core theme. I also think that the ice golem's frost core should be renamed to something a bit more ambiguous to avoid confusion with the other cores.
Lastly, the biggest thing I've started wondering about is the situation dealing with the mechanical bosses. They quite obviously fit the theme of the Cyber biome, seeing as how the biome is a shining utopia of robots and the mechanical bosses mirror this. However, what happens to the mechanical bosses in a hallowed world then? They just don't seem to fit as well in such a world, since the theme of mechanical robots is lost in a pink world of rainbows and light. Before this suggestion the mechanical bosses were more of a unique thing, but now they fit too well with the Cyber for the Hallow to use them. I don't know what could be done to improve this situation; maybe making Hallow reskins for the mechanical bosses? I'm not too sure but in my opinion this is a pressing matter.
In conclusion, great suggestion even if I do have my own personal opinions, and I hope to see more in the future! Maybe one day this will get added to the game and everyone supporting this suggestion can celebrate by fighting a few Motherboards