Ok, this thread is enormous, and thus I can't know if anyone has suggested this before (I'd wager that yes, they have). But my opinion is to make the Assembly Hall as an outpost for the three dwarven factions, so they would have three options just like Mordor. Dale/Esgaroth, Assembly Hall, and normal Outpost.
I think taking away Dale/Esgaroth from any one of the dwarven factions to put the Assembly Hall in its place would be terrible, since that would take away the only decent archers they have. And it's hard enough to play a defensive game as it is.
Well, there are many reasons why your suggestion is bad, but mainly there are Lore reasons. Dwarves don't work like Mordor, you simply choose at start which faction you want, and in that kind of gameplay 3 different outpost for any of those kingdoms are too much and impossible.
First and main reason are lore and timeline.
Iron Hills faction has currently Lake Town as Outpost. This is not Lore inappropriate, because there are not any single link between Lake Town and Iron Hills in lore!
Second reason why you preposition is bad is because current Dale Outpost are from time of Erebor, that is point in timeline after currently implemented Iron Hills, so there are no logic reasons for that. (Same resons for combination of Ered Luin and Dale!)
Third Iron Hills already have main Bow hero, Drar, they don't need Bard or any kind of extra bowmen as units.
And mainly three outpost for any of dwarven kingdom and that means 3 new heroes for each of those kingdoms. This is also anti lore and anti TIMELINE (Brand and Bard at same time?! They are separated by 200 years, Bard is dead and long time after that Brand is king of Dale!) And for last reason this is OP idea.
Assembly Hall is firstly suggested for Ered Luin (idea from movie), but later we noticed that Lake Town are most fitting for Ered Luin and Iron Hills don't have appropriate Outpost so this preposition about Assembly Hall moved naturally to Iron Hills (there are also reasons from Lore, you can read all about this in some of earlier comments, I really don't have intention to write all that now.

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And I know, as you said already that this is big and enormous topic, but simply you should always try to read most of it, simply you can't understand idea in general if you don't read all about it.

That is just my opinion about ideas and topics in general...
I would quote this:
I think taking away Dale/Esgaroth from any one of the dwarven factions to put the Assembly Hall in its place would be terrible, since that would take away the only decent archers they have.
And you really think this (I would say epic idea) is bad becuse of archers???

And you really think that Lake Town's archers are needed to Iron Hills? I almost always play without Outpost (Lake Town) in general, I believe that most players don't use those archers as main range units besides that Iron Hills archers who do insane amount of damage with fire upgrade or battle-wagon support. And they are available from early start of game.