What I was thinking of doing was when I hit those big numbers (100, 1000, 100,000) subs to go out and give x amount of youtubers a high five. For example, If my special number was 250 subs, I would make a video of going out and giving 20 (or whatever number you choose) youtubers a high five.
This is how I would shoot it. The youtuber you are going to give a high five to is recording their own show. You walk in on screen, they turn around and give you a high five and then they go back to what they were doing. You just walk off screen to another youtuber, etc... Put that into some kind of compilation and then you have awesomeness.
But I'm probably not going to do this because I don't like dealing with people, so if you want to take the idea go ahead.