JAX-LAX could happen again. It’s JAX’s largest market that isn’t served (which considering LA is the number 2 market that isn’t saying much). But, apparently JAX is LA’s largest unserved market according to JAA about a month ago. Delta tried the route in the 2009 timeframe, but it failed-couldn’t full the plane. They used a 737, which I believe is the smallest plane they’d put on the route (though now they might be able to use an A319 now that Northwest has long been integrated. There is also the decision on whether or not the LAX-JAX leg is a redeye like Delta tried. Personally, not a fan of a domestic red eye (the flight isn’t long enough to me).
The only way I see a JAX-SEA route is if Delta does it (SEA a Delta hub and is one of their Pacific gateways) or Alaska enters the market. A Seattle destination alone wouldn’t do it; there would have to be some connecting traffic. Though, there must not be a huge market in JAX for those connecting to smaller western cities, or Delta would try a JAX-SLC flight, which is a much larger Delta hub and a lot more logical for folks connecting out west.