The future plans keep changing. All that is certain is that come finished southern extension and new tube train ordering time, they will need to be a bit more definite in order to acquire the correct number of trains. There is little doubt the decision will be affected by the completion (or not) of the Met extension to Watford Junction.
In its simplest terms a 15 minute service off-peak north of H&W is probably OK at the moment, but who knows in the future, but is inadequate in the rush-hour. The line needs a boosted peak service. Assuming there are no extra slots at Euston for LO to run a peak service, this means re-extending the Bakerloo. The problem is that at the moment every LU train is committed to operating the current peak service, there are no spare trains that could run a Watford service, and there's no way LU are going to decrease their service in central London, that would certainly be a big NO! That's why no change is possible until after new trains arrive, there aren't enough of the current ones.
The future service north of H&W would only really be needed by 3 stations - Headstone Lane, Hatch End and Carpenders Park. Bushey is also a London Midland stop, Watford High Street will also be covered by Met extension and Watford Jcn is covered by both. Without population projections for all of these it is difficult to guess the required service level 10-15 years into the future. The most likely solution will be LO terminates at Willesden Jcn and Bakerloo is re-extended to Watford Jcn (probably 6tph all day). Then you could have up to 6tph reversing H&W (unlikely to need that many - but possible). You then most likely have no need to run Stonebridge Park reversers other than stabling trains inter-peak. But a cheaper off-peak version could be 3 tph to Watford Jcn, 3 tph H&W and 3 tph Stonebridge Park. It will all depend on the travelling public's needs.
What will be seen as essential is getting enough trains to cover the southern extension and to ensure a 2.5 minute or better peak service south of Queen's Park. At the moment the peak service is 2.5 with the odd 3 minute gap because there are insufficient trains for better!
I'd love to see the Bakerloo get both north and south extensions and enough trains for a 2 minute (or maybe 1.5minute) peak, but I doubt it will happen because for reasons I've never understood the Bakerloo is like the forgotten middle child of LU. It gets the hand-me-downs from everyone else (the new young up starts that go better than it (Jub)). The neighbouring mature, older lines that go faster in the peaks (Met). Of course the southern extension may massively change its significance and passenger usage - let's hope so!
Also people often forget there's a peak flow into/out of Watford at the top end of the line as well.