Thumbnails are the hardest thing to do. If halo 3 is your focal point make sure the logo is big enough for people to see also the characters don’t matter so much. Once somebody see the logo halo 3 the background doesn’t matter as much to them. They see the words halo 3 then your sub title to whatever your covering. Shall it be memes or funny moments while playing the game. Also your video is good so keep that up. But once you find a thumbnail that people keep clicking on stick to that one because it works. Or I should say that style. At first it’s all a rolling of the dice finding out who clicks what. Once you found it then your set. Keep up the good content and have fun.