Qualifications/rules for a taxi position in a keeper league

[Adidas95]

We're in a 6 player keeper league but we want to add a 7th keeper as a developmental, sleeper, "taxi position". I also want to make sure there is no way someone can exploit it as a 7th keeper; I want to make sure its a developmental position. Right now all I can think of is it has to be a first or second year player. What should the rules and qualifications be for the taxi position?

[RB6]

Yes, it should only be for rookies and sophomores. Though, I would prefer it's rookie exclusive. There should also be a rule that (expected) productive players (ie Davis, Fournette) should not be allowed on taxi, to discourage tanking. Maybe a rule that players drafted in the first round of rookie drafts must be rostered. And once a player is promoted to the active roster they cannot be sent down to the taxi squad again.