Well to change costumes you must either, beat the game on Normal, or, play the challenge maps. Secondly the game is more of the same with just a little new thrown in, but it's good, not boring. I think it was wise of them NOT, to stray to far from the original mechanics. I think it's better to be safe than sorry on your first outing, and have people say they wanted more. This gives good feed back and, they can incorporate what people wanted, and wished was there in this game for the next Arkham game. I wish they would have left out the multi-player though. I don't really do the multi-player, and I don't understand why the developers thought that this game needs multi-player. They should have forgot that and added more to the main game, maybe more side missions, tweaked game play, things like that. Overall it's a good game, and WB Montreal have set themselves up for future success if they continue with the Batman franchise, which I hope we see many more of!
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Re: the DLC, that's something under current investigation.
-Network-Support/Batman-Arkham-Origins-Deathstroke ... is the current support thread, and:
-Feedback/Missing-Deathstroke-DlC/m-p/5255#M378 is where the developer is collecting information about the issue.
I have received news that the Deathstroke DLC should be available now for users that have pre-ordered. The DLC may need to be added to PS Store Cart so that it ends up on your download list. Hope this helps!