Simple really. Now play a 14 week schedule and have playoffs weeks 15, 16, and 17. Still have week 18 where players might sit, which was week 17 last year.
I think it is this simple for the most common 12-team league formats. There was most likely some kind of schedule imbalance for H2H matchups. For a lot of leagues without divisional formats it just came down to a random schedule draw where you happen to play two of the teams in your league twice. I guess you hope that it balances out so there isn't one team playing two of the heavyweights twice and another team playing the two guys who forget to set their lineup. There's really no point sweating it out when nothing is guaranteed at the start of the season when the schedule is generated. These leagues are just adding a third team that you play twice. No sweat at all. If anything the third (2x) team makes it more likely that everyone gets a somewhat evenly balanced schedule, assuming that there are owners in this league who are historically more likely to be found at the top and bottom of the standings
I think it becomes more of a talker in a 14-team league, where you're losing a perfect round-robin. Or a 10-team, 2-divisional (round robin & round robin x2). In a 14-team league, you still play everybody once, but now there's ONE team that you play twice. There's something about this that does raise some question about how that one team should be determined. Yeah, you could just go random schedule draw and call it a day. But some commissioners who probably spend too much time thinking about this stuff might see it as an opportunity to add a wrinkle to the league that could make it a bit more fun for everyone. I think a lot of the options have already been put forward.
1. global high score / competition week - like the take that it gives Toilet teams incentive to keep managing their teams late in the season
2. set rivalry week - another game against the owner you have the biggest blood feud with, what's not to like?
3. take a page out of NFL parody, strength of schedule model. Best teams from last year play the best. Worst play the worst.
4. Last week of season, Playoff Parody Flex Week - same idea as #3, but based on current standings to maximize week 14 seeding and play-in drama.
5. Top/Bottom Flex Week - Top half gets wins. Bottom half take the L. Start the season with the result that you deserve.
6. Another one that's out there. Add an additional round to your live fantasy draft. Let owners draft the other owner they want to play twice. Also available in auction format. Think about how sweet it would be for the perpetual bottom feeder to get called out by a team that passes on a top kicker or defense just to play the worst team twice, only to have the tables turned on them with a two-game sweep.