I agree with @vengefulpickle on this one. I can't think of a time when I'd willingly declare first, and even if some situations exist where that option would be beneficial, it's a lot simpler to just say that the person with the most life declares first.
On a different note, I've compiled a list of Arena Cards (I like this name over Map Cards, but I'm not changing the thread title yet) based on suggestions people have made. I'm not posting it yet, since people aren't voting on the poll (seriously, if you're reading this and you haven't voted, please vote). However, I can tell you that there's already 17 of them in my personal list, and that's leaving out some!
To everyone who's contributed: Thank you. I didn't expect so many great suggestions so quickly, and it's blown me away. My only complaint is that I'm going to have a really hard time cutting this list down to 12 (the same number as in Pandante and Codex, leaving aside the Kickstarter exclusive Casino Cards). That's a good thing, though -- it's a way better problem to have than too few!
Keep them coming, everyone!