I don't know how the ESPN leagues work either. If we have twelve teams next year, will we automatically have one elimination round in the playoffs? Does the league allow making changes like that? Will it allow us to have the final game on week 17?
I was playing around with custom league settings and it seems quite flexible. You can choose the length of the regular season and the number/length of playoff rounds, among many other things. With 12 teams you can have 2 or 3 divisions for example, and set up playoff byes/wildcards accordingly.
I would suggest to keep it simple though and not get too crazy, but if the preference is to not use week 17/18 then we would have to either make playoff matches 1 week long (might be too harsh), or shorten the regular season. For example with 12 teams you can probably make it 11 weeks and play each team once. There are also double-header options but that seems a bit weird (you play two teams at the same time). Maybe it would work on non-bye weeks. We'll have the long and boring off-season to figure this out.
It is funny, the season is not over yet and we are already thinking about the next FF league.
The first thing we need know is how big the league will be, I think it is pretty much a given that at minimum next year we will have 10 teams, with the possibility of reaching 12 teams if the demand is high.
In the case of a 10 Team League we could create 2 divisions of 5 with the members of the same division facing each other twice, as it is the case in the NFL, so the regular season will end on week 13, if only 4 teams move to the playoffs then we could use a 2 week period for each matchup and the League would end on week 17, or we could allow 6 teams to move to the playoffs with the top 2 teams having a BYE week and the league ending on week 16.
Also it could be interesting to change the order of the tie breakers, the current order while it is the most straightforward also takes some fun out of the game, it could be more interesting for head-to-head matchups to be the first tie breaker as it is the case in the NFL as well.
But as you say we have a long time to discuss this and find some common ground.