Think you have misunderstood my suggestion slightly (maybe the way I described it). In the bulletin today they mentioned that there have been many feature requests for the coin jar (for example) this included being able to round up to the nearest 50p etc. They then discussed how you could do this already but you would need to be a developer and be able to use the API.
The suggestion I had would be to create a user marketplace which allows people to develop these types of saving strategies. This would then allow all customers to be able to implement the strategies on their own account. Ideally this would just be a 1 click “install as such”
The possibilities would be fairly endless:
Ability to create a strategy that fines you (fine goes into the coin jar) if you buy fast food
Ability to round up to the nearest or next whole £
Ability to limit how much is saved per month
As Monzo already have your financials the marketplace could also have the ability to calculate roughly (providing spending stays the same) how much would be saved monthly.