Effectivity:
SCRUM and other advanced productivity systems:
Productivity systems such as the SCRUM board and Kanban board for companies and teams are well developed and could be adapted to personal productivity
Machine learning:
Over time a neural network could learn your productive strengths and weaknesses and optimize your reward system to help you overcome these. (Extra points for being productive early in the day, on weekends, or for extended periods of time, etc)
Arbitrary reward system and psychology study implementation:
There are many studies published regarding systems, habits, and tricks to help with productivity. An arbitrary point system in which you are rewarded with a meme, affirmation, or even small in-app currency (example: SweatCoin).