I've been pondering this idea for a few days, and I can't think of a downside. It would take a bit of graphics work, but I think far less than adding new armor sets.
You can probably guess the idea -- simply let us choose to wear headgear from our primary or secondary class. This would allow a bit more tactical diversity -- do I want more armor on 12.5% of hits, or a bit more energy, etc -- but not enough to necessitate any major rebalancing. It would give us a little more ability to customize our appearance, which I think a lot of people would like. That could also be useful as an addition to the game's "body language" -- an Elem/Monk could switch hats based on whether they want to be an obvious healer (and thus in high demand, but quickly targeted in pvp) or not.
What do y'all think?
Easy idea for more armor diversity -- secondary hats!
wingedcoyote
Zorax
In every RPG I can get my hands on I look for wide brimmed, pointy hats. I have to chalk down GW as number ALOT without one. 
I would actually like cross-profession outfits without stats that you can customise with a new item called "Elementalist rune for robe/hat/boots/..." or whatever your profession. This would have the stats of your normal profession's armour, and then you could assign it to a cross-profession outfit. This would be more expensive than buying a normal profession outfit, since you have to buy/find both a rune and the outfit parts. But would be cooler. Some outfits would have limits, though. That way you could have a wide selection of parts to customise your look with, and hopefully including pointy hats. This means that you would see necromancer with face masks, elementalists with helmets, mesmers with pointy hats, rangers with sombreros, etc.
I would actually like cross-profession outfits without stats that you can customise with a new item called "Elementalist rune for robe/hat/boots/..." or whatever your profession. This would have the stats of your normal profession's armour, and then you could assign it to a cross-profession outfit. This would be more expensive than buying a normal profession outfit, since you have to buy/find both a rune and the outfit parts. But would be cooler. Some outfits would have limits, though. That way you could have a wide selection of parts to customise your look with, and hopefully including pointy hats. This means that you would see necromancer with face masks, elementalists with helmets, mesmers with pointy hats, rangers with sombreros, etc.