But, and I think i mentioned this before, this is explicitly stated to be a
different kind of Thor. Picking up the hammer doesn't mean he has to act just like OG Thor, (especially considering this is Ultimate Mjolnir, so may have somewhat different views on worthiness) he's not going to be doing things in the usual Thor way. The usual Thor way has proved to be woefully ineffective against Malekith so far in terms of actually stopping him, so it is time to try something new. And whoever it is, maybe they realize their own usual methods are also going to be ineffective as well, (Volstagg on the council has accomplished jack all, Loki's accomplished nothing as a potential double agent, etc.) so they are just adopting the persona that is NEEDED to get things done. It's not so much a matter of want as a matter of need. As the god of stories, Loki may be able to take a step back where others can't, to see what type of character is needed here.
I don't mean to come across as me being married to the idea of it being Loki, though I know it may seem that way. I just am trying to say I do think he is as good a possibility as anyone else right now, and shouldn't be shot down out of hand. Some of the arguments brought against him in this thread also shoot down every other character people have suggested. (colour scheme is wrong? well guess what that colour scheme doesn't match ANY character in the story so far, except maybe Cul. Has to be someone who was previously Thor? again, excludes every other possibility as well) Some things DO fit. I truly do think the dialogue fits, the scale armor is his style, the black chainmail looks like black hair, the wings on the helmet are Loki style, they brought Thori into the story, (wanna bet he can recognize Loki by scent?) the narrative arc would make sense as a continuation of his story in the book, he has an actual reason to model his disguise after Odin, given their troubled and complicated familial relationship, cheeky teases in the letters pages and solicits, and so on. I know there are some iffy aspects to it being him, things that may not totally fit, but there are iffy aspects to every other suggestion so far as well, no character mentioned is a 100% perfect candidate, barring some future explanations and reveals. At the moment, my favored candidates are him, Volstagg, and after taking another look at some things during this discussion, Heimdall.
*edit - I don't personally follow Dr Who, but a friend of mine mentioned the whole 'War Thor' thing may be a
Dr Who reference, for what that's worth.