So I figured I'll start to throw in my two cents here and there, beginning with! Bahamut.

From what I've gathered from reading the forums is that there are two problems with having Bahamut as an summon. 1) He's too big to fit into most if not all inside areas and hell, even some outside ones too. 2) SE not wanting to knock Bahamut off his pedestal of high power and thus creating a less powerful version (aka capable of being defeated by a rabbit or worm) just so Summoners can have him and not make him overpowered.

These are total BS arguments.

Starting off with the first "problem," Bahamut's size is only a problem if he's an actual avatar and not a 2 hour (more on that later). If he were implemented in avatar form, having a full blown Bahamut following you around would be impractical. So this is where we(SE) would use our imaginations (Ridiculous!) and think up creative solutions (Even More Ridiculous!). The first one that comes to mind would be to simply shrink him to the size of, say, Diabolos or possibly slightly bigger. Although having a Pixie sized Bahamut follow you around would Never get old. I suppose the problem with that would be "Why would he be smaller than his actual form?" and this can be addressed in oh so many different ways. There would undoubtedly have to be a quest or some sort to get Bahamut as a summon so it would be easy to incorporate it into the story of the quest. Such as only being able to harness a fraction of Bahamut's true strength and the physical(/magical?) manifestation of that power is quite literally an avatar of that portion of Bahamut's power. Or it could just simply be that Bahamut can change size at will to fit inside of places (Hey, even Bahamut thinks it get a little chilly outside too). I'm sure there are plenty of other ways as well that other people can come up with, this is just me thinking about it for a couple of minutes and I'm sure I can come up with more later.

Now, if Bahamut was implemented as a 2 hour size would be completely irrelevant. His ability would no doubt be Gigaflare and the only part of Bahamut that would really need to be there is his face. Which makes this totally doable, I've seen plenty of things (including dragons) crawl out of a seal that appeared on the ground. All you would need is Bahamut's claws come out, then pull up his head and then use his laser face. Super easy. The only problem with Bahamut as a 2 hour is that you don't really want another Odin (assuming he stays as a 2 hour), so we(SE) would have to figure out what Gigaflare does other than just straight up damage. I noticed in another thread that someone mentioned some debuffs which could work, or it could just be flat damage that was significantly better than Odin but only available to level 99's.

I sorta touched on the second "problem" which is not wanting to make him less powerful to make him balanced as a non 2 hour summon, but the fact is that is just doesn't matter. He's an avatar, he's not actually Bahamut. We're not going into the BCNM, throwing a collar around his neck and going, "Okay follow me you're mine now." To be honest I don't even think we're doing that with the Celestial avatars either, we're fighting their physical(/magical?) manifestations (aka their avatars) defeating them, then obtaining the ability to summon them in avatar form. I don't see how this would be any different with Bahamut. You could also just make it so only level 99's can summon him completely eliminating the 'being defeated by a tunnel worm is too humiliating' anyway...

On a quick side note... Please! For the love of Bahamut let us have Odin and Alexander as Actual avatars, I did not do all that work of going through ToA to get two situational, AT BEST, "avatars." You realize we ONLY use Odin to kill regular mobs in abyssea? It's a complete waste of two potentially Awesome summons, one of which is a personal favorite of mine. /rant

tl;dr
The reasons we've been given and the arguments made for not having Bahamut are total BS, if they don't want us to have him just SAY so and don't hide behind these stupid reasons.

So I don't know, I suppose this is where I say, "So, what do you think?" and "No disrespectful language." /shake finger