After you get all 5 Gold Stars, Go back and Fight Crimson Dragon. After you defeat him and watch the credits and cutscences you will recieve a Black Ace Wallpaper that you can set as a background for your Hunter VG.
Fight RogueZZ
At the title screen, hold L and press G Comp Star, M Comp Star, M Comp Star, SS Star, G Comp Star, S Comp Star, and Black Ace Star. He'll appear in the Meteor G Ctrl CC, which is located in the Orbital Base.
G Comp Star
Collect all the Giga cards.
M Comp Star
Collect all the Mega cards.
Make Sigma bosses appear randomly
At the title screen hold L and press these gold stars in order: S Comp Star, G Comp Star, S Comp Star, M Comp Star, SS Star, SS Star, and Black Ace Star. You should hear a confirmation if done correctly.
Mu Star
Beat Rogue ZZ.
S Comp Star
Collect all the Standard cards.
Sigma Star
Beat Sirius Sigma, Moon Destroyer Sigma, and Apollo Flame Sigma.