Keeping in mind what we know (that Moffat will obviously be writing, that we seem to have a near-contemporary companion, that Gatiss is likely going to write another episode, that Mathieson has said he has an idea for another episode, that the structure of the series will differ once again), and tallying it up against what I would want, I wouldn't mind a series that shaped up like this...
1 - Standalone, by Steven Moffat
2 - Standalone, by Jamie Mathieson
3 - Standalone, by Sarah Dollard
4 - One of Three, by *new writer*
5 - Two of Three, by Jane Goldman
6 - Three of Three, by *new writer* & J Goldman
7 - Standalone, by Mark Gatiss
8 - Standalone, by Toby Whithouse
9 - Standalone, by Catherine Tregenna
10 - One of Three, by Steven Moffat
11 - Two of Three, by Steven Moffat
12 - Three of Three, by Steven Moffat
It caters to the 'new format' again as we've not seen three parters outside a finale before. The first three-parter could hypothetically be two standalone episodes that then converge into a single story come the third episode...if the writers had a decent idea for it. Moffat could have command of the home stretch given its his final year too, and the perfectionist in me counts eight writers there (Moffat's series have all alternated between seven and eight each year so far) - with one new one thrown in and a few who are relatively new still.
The Bermuda Triangle ideas which I love would cater to a three parter like that brilliantly. I also think some more standalones is crucial... Series 7 went too far with standalones, Series 9 didn't go quite far enough. Series 8 nearly got it right but its stories just weren't strong enough without a big two-part event in the middle or something. It's all about balance.