As an owner, this would be great. I've spent thousands of flex on my team, would be nice to get the other guys to pay for the friendlies (which I'm sure they will

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IMO don't go overboard on "what if the team sells." Let an Agent see how much flex is in the bank before donating so teams don't go overboard, and enforce a "once it's in, it's lost" rule for the bank.
Also, need to ask the question - is this only for scrimmages.
And if so, that isn't much for a flex bank... Could think of more flex options for teams, such as subforums for their team forum (50 flex), scrimmage offense vs. defense, GLB sanctioned tournaments (every team puts in 200 flex, winning teams get % of the flex bank; just have to watch out for collusion I guess), new stadium options, etc.