What if......and I'm just spit-ballin' here.......they go for a premise where the space needle is like a giant "radio" tower emitting sound waves. The bottom supports could rest on soccer balls, with a killer whale jumping out of the bottom "negative" space. Coming out the whale blowhole is a spray, with a soccer ball resting on it. The center stanchion for the space needle/tower is navy blue, and decorated on it could be letters for the team's name: S-E-A-T-T-L-E S-O-U-N-D-E-R-S FC. Focus is on the colors black, white, and navy blue, trimmed in the action green. IN the background of the logo is a single shield in the "loose" shape of the NFL (or MLS) logo in black and white.
Or...go with a medieval-style coat-of-arms logo shield, bisected diagonally. The top right in navy blue background shows a killer whale leaping over a soccer ball. The bottom left in action green background, shows a silhouette of the space needle in white, trimmed in black. The diagonal stripe can be in either white with black separation or black with white separation. Over the shield, spaced to fit corner to corner: SEATTLE SOUNDERS FC.