Couldn't the game use the formula that it has for the 'Best XI' that it creates automatically when you make a team? Basically just run that for the rest of the team after fixing your position.
If you added a little bit of a degree of randomness to that between matches, or factored in international selection - you could probably get close to 'dynamic' lineups - at least as close as you can probably get without going the full way by including generated players, retirements and skill pro/regression and injuries for all players.
If you added a little bit of a degree of randomness to that between matches, or factored in international selection - you could probably get close to 'dynamic' lineups - at least as close as you can probably get without going the full way by including generated players, retirements and skill pro/regression and injuries for all players.