Re: Lingering Will to join in the final battle?
It's not like there is an underlying metaphysics that the writers and designers are confined by...they can do whatever they want.
Except that there are underlying metaphysics that have been more and more fleshed out as the series went on, by in-game statements,journals/reports and Nomura interviews.
If this would not be the case, we would not have any basic concepts to base most of our theories on.
Like Spockanort said, the high accurracy of some theories here comes because those are based on already established rulesets we know so far, even if some of those still have some ambigious or mysterious parts that aren't fully understood yet.
To say "anything is fair game" and just shooting blindly into the blue has the high danger of being just wishful thinking rather than a true theory because it cannot be grounded on anything the series' universe has shown us so far.
For example, linking Eraqus with the Lingering Will is not only something made up out of thin air, it also openly contradicts what the Lingering Will itself has already been established to be: Just a pile of clunky metal animated by some leftover thoughts by
Terra.
Well from what I understand, it isn't anything that can grow a heart, but any body, and a body would consist of all the memories, experiences, etc. I doubt that lamp that sat in Sora's room had a heart...that would be just awkward if every grain of sand in KH had a heart so Sora is killing sand as he walks on it <__< The armor is a thing, just like a lamp, but it is controlled by Terra's will. Terra's will isn't a body, so terra's will cannot have a heart (as far as has been stated thus far).
Seems like you haven't paid enough attention to Xemnas, Ansem the Wise
and Nomura as they all pretty much spill out that
anything can grow/create a heart, even a tree, a rock or "petals on the wind" as Ansem the Wise put it. If growing hearts was just possible for bodies (as when you define "body" being a biomass of blood, muscles and flesh), beings like Tron, Data-Sora or Pinocchio could not have a heart because the former two are made out of "data" and the latter out of wood. Yet they are implied/confirmed to have one anyways.
The main condition for a heart to grow and be nutured however is:
Ansem the Wise said:
The heart has always been quick to grow. Each exposure to light, to the natural world, to other people, shapes this most malleable part inside of us.
Joshua said:
By ourselves, we're no one. It's when other people look at us and see someone--that's the moment we each start to exist. All they needed was for someone to see them, connect with them. And the two of you were a big part of making it happen.
Mickey said:
I'm not sure, Sora. All I can say is that something in you has changed. The Keyblade takes its power from the wielder's strength of heart. But an old friend of mine once said, "A heart is so much more than any system." I gave you the Keyblade as data, and never expected it to match the real Keyblade's power. But during your journey, you made a connection...with us. You've risen above the system, Sora, and gained power unlike anybody else's. And THAT's what brought your Keyblade back.
The connecting with the world and other people, interactions, forming memories and an own sense of self, that's what ultimately results in a growing heart.
To take up your "lamp" example, it may certainly be possible for it to have a heart, especially if there would be someone who truly loved and tended to that lamp (however ridiculous that may sound), although since a lamp normally doesn't have sentience, it's heart probably won't grow fast nor very far.
Keep in mind however that Pinocchio also once was just a wooden puppet, not much different to a wooden lamp except in physical form.
So for the Lingering Will it isn't so much that it is impossible for it to grow a heart, but extremely unlikely because it has close to no interactions with anyone over the course of it's ten year existence. Even in the final battle in BBS, the only one speaking is Terra-Xehanort and I doubt that counts as "connecting" so much with others.
Well they did kind of use the concept of a Lingering will with the Mentor of Light and Vanitas' Lingering Sentiment, at least I think they did, so I agree that it's a cool concept that should be re-visited. Not to mention if Terra's armor isn't at least found in KG, I don't know how they're going to explain his sudden disappearance given he was last shown to be there in BbS (and KH2FM, which even if non-canon, he was there in BbS).
They may be created on the same concept, but do not forget that the Lingering Will of Terra is in fact a canon entity in the series, while Vanitas' Remnant, Armor of the Master and No Heart (the latter two being confirmed by Nomura as data simulations) are not.