If he doesn't want to change the current monthly system, I think another way of mitigating the short the length of the videos, would be if he alternates between his original (Jackerverse) characters and already existing characters for the voting process.
Example: one month supporters can only nominate/vote ideas for Jack's original characters, the following month they can vote any non original characters such as: Dexter's mom, Helen Parr, Lara Croft etc.
This would increase the chances of previous videos getting sequels like Southern Hospitality, Bedtime Boogey, Jenny etc. resulting in a kind of "long projects" split in mini parts (monthlies).