If you hold the 'All Kick' button as soon as you see 'Now! Fight your Rival!' you will be matched against your character's second rival. This only works for Cammy, Ryu, Chun-li, Seth, C.Viper, and Guile.
Gouken will appear after the battle with Seth in Arcade mode if you play with the default settings and do not lose a single round, perform 5 Super or Ultra Combo finishes, have 2 Perfect Rounds by not getting hit, and execute 10 First Hits by connecting with the first strike in a round.
If you play Arcade mode in standard best of 3 and win at least 1 'Perfect' match which means not getting hit either round and not using 'Continues' Akuma will challenge you after the final battle with Seth.