I'm not sure that a new map rotation would fix the problem of people not playing siege, but it would at least begin to address the issue somewhat.
I think a big part of why a good number of established players stay away from NA2 most of the time is because the battles there are not very dynamic compared to those you'll find on NA1. When almost all cavalry builds and a large number of ranged builds are useless on the majority of siege maps and players with high agility are handicapped on a large number of siege maps, they inevitably gravitate towards NA1 rather than NA2. The presence of pretty much only infantry on siege means that every match will start to feel the same to your average melee player and he will most likely get bored and leave after a few rounds, especially if he is having trouble sustaining a multiplier. It's basically the same reason why every single melee-only server to date has failed.
Try introducing some maps that allow all classes and builds to contribute in a meaningful way (as battle does) but aren't just "here's a flag on top of a big hill" or something like that and see if the population on NA2 increases at all.
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