Basically an event where SK hires a band/composer/etc to do a one time event. The missions and levels that come with that event have that particular band's music or songs. Costumes and items reskins would be based upon various things from the band.

Special Tokens or materials could be gathered from these levels and used to craft new items.

A reskinned Cold Iron Vanquisher made to look like an Electric Guitar.
A reskinned Fearless Rigadoon made to look like drumsticks (dual wield appearance like furious fork).
A reskinned Irontech destroyer made to look like a Drum set.
A reskinned Omega Shell made to look like a Gong.
ETC.

Promotion Boxes with costumes that resemble signature looks of various musical artists (KISS stylized jelly armor anyone?)

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This would be a good way for SK to make some money off of promos and would help smaller bands get their name out to more people.

AQW is an MMO that has done this before and it works quite well.
http://www.aq.com/events/arcattack/
http://www.aq.com/events/tmbg/
http://www.aq.com/events/ayi/

There are several more but I won't list them.

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Missions could be rescue the band, or collect our lost stuff (instruments, etc).

The Band for each year could change and bring new items and promos every year.