Regarding the tri-series format in the past, it has generally comprised of 12 ODIs during the preliminary round + best of 3 finals. However, there was a shortened tri-series in 2004/05 with only 9 ODIs during the preliminary round. In 1998/99, there were 15 ODIs during the preliminary round. With the advent of T20 cricket since then, I don't think we will ever go back to 15 ODIs.
Also, this is controversial but I know a lot of you will agree with this point: we shouldn't let countries like India screw over our schedule. We generally finish our international season in early February but because India came out here later, we don't finish until early March. It will be beginning of NRL season by then, and also kids have been back at school for over a month. It's going to be very hard to get ppl to the tri-series games that aren't held on weekends once school starts.
I've said this before and I will say it again: I have no problemwith 12 ODIs, but what I would do is that the games involving the neutral teams should be taken to regional areas. This would be the non-Australian game of a double header played at WACA, Adelaide Oval, The Gabba and Blundstone Arena. This would be the perfect opportunity to take the game to non-city regions and with proper promotion, would sell out and get more than just a few thousand that would turn up at those test venues I mentioned. Newcastle, Canberra, Albury, Launceston, Geelong, Gold Coast are 6 areas where I'd love to see international cricket played at.