Doing a forum search this topic came up a decade ago (e.g
(186483.1),
(223178.11)).
We have a transfer deadline for the league and for BBM, so it seems like it would make sense to have one for the cup as well. I'm guessing the reason we don't is the difficulty of implementing it, as different cup competitions end at different times.
Being able to "rent" great players for low transfer fees is partly the issue, so perhaps something like a minimum transfer fee for players in general. For example a minimum price of 2x salary, so a $180k salary player would be listed at $360k. This would have impact on other areas of the game so would need to think more about it.
Any thoughts or ideas on this subject?