Spot on with the acceptance criteria. I managed lineups last year in excel and it was a nightmare. I'm going to write a little program of my own for this summer's season if this isn't implemented (as this is 4 years old, I better get coding).
In my mind, the features that would need added:
- Use game roster (as you said)
- Available positions (custom formation - softball in my case, so like pitcher, 1st base, etc)
- number of rotations, or add a rotation (innings in my case)
- needs to support X benched players
- reporting on how many times a player has played a position would be a plus (for a given season)
- option to randomly assign players to positions and allowing rearrange would be a plus

Line-ups would also be very helpful for our rowing team.

Currently we are managing 60+ kids in a u10's rugby team in Canada. When we play games (i.e. tournaments) we have to enter 5 or so teams from our selection of available kids. These teams although playing at one location could be spread quite far apart. Would be good to have the line-ups option to assign kids to "team1", "team2", "team3" etc. with each team having a set of instructions or details such as who the coach is, where to meet etc. Right now, I'm managing part of this (or at least planning on doing so) by creating duties for each game. This means creating 10 duties for Coach A, 10 duties for Coach B etc. and assigning kids to one of those duties. It starts to get messy with 60+ duties to assign and I guess technically you could make a mistake and assign one kid to two "duties" or teams accidentally. Maybe the best bit would be to expand the roster section and instead of having just on or off, maybe have multiple options in a dropdown (definable for each "game" or templated), e.g. team1, team2, team3 etc. Then having a way of viewing players by roster would complete that piece so a coach would know who was on their team in a particular week. Sorry to be verbose but rather than providing just a problem, I like (where possible) to offer a possible solution.

It would be great to form my team and formation to publish to attending players on TeamStuff prior to the game.

Great suggestion. Ability to have multiple line-ups is key as int he younger age groups in US recreational soccer you often split the team into two and play two games concurrently. I would want to be able to do this and and then change things up, week to week.