According to Mike Garafalo's Fox Sports report, and subsequently confirmed by Bob Condotta, officials are considering adding 2,000–3,000 additional seats on the 300 level at the south end of the stadium, where there is currently a break in the 300-level seats. This would bring Centurylink's capacity to somewhere between 69,000–71,000, depending on how many they add and the configuration of the additional seats/standing room tickets they can currently sell.
The first impact the Sounders could feel would be construction woes, though I am not enough of an expert to speculate on how long construction would take or even if construction in one section of the upper deck would have any significant impact on Sounders matches. It is a question to be answered, though.
The biggest immediate positive? The Rave Green faithful would have an opportunity to crush the MLS single-game attendance record (which excludes double-headers) set in the league's inaugural game between the LA Galaxy and the New York Metrostars of 69,255. The Sounders have made crushing attendance records passé, but this one is more or less the Holy Grail of attendance records. If the Sounders could manage 71,000, they could also make it the 5th highest attended MLS match of all time, beating all but four double-headers.
While the Seahawks' run of completely sold-out, packed and loud crowds certainly is the primary driving factor of proposed expansion, it is likely that the Sounders are a significant factor as well. The Sounders' average attendance has increased every year, and Rave Green-clad fans have shown they can completely sell-out the stadium under the right conditions. Those few-thousand seats would be a welcome addition for the men in Shale, as they certainly would be for the men in Wolf Grey (if they'd actually wear those sweet uniforms at home, that is). More significant, though, is the possibility that the new section could be sold as a block as a cheap season ticket area, similar to how the Hawks Nest was recently opened for Sounders games. It could provide a natural expansion for the Sounders' seating chart in a well-defined and logical area.
Keep in mind now that this is being phrased as under "consideration." But if it happens, you can bet those seats will be filled with Rave Green kits before long.