Actually if you're afraid of biasness, setting the minimum requirement to something like the top 5 in most stats all time and a season would be enough to prevent that.
It's my 9th season as U21 coach, and I've only had 1 U21 player make top 5 in those two categories, and that player isn't even my own. For the record, the current U21 player is the 7th player in my team to make the U21 (by merit of course) and none of them meet that criteria. So even if I wanted to nominate all my former players into the HoF, none of them would even be eligible.
Add that to a limitation of 1 nomination per term, you won't really have to be worried about biasness or the HOF being so bloated that it becomes worthless.
The only drawback would be that I, someone who came in a few seasons ago, would not be too familiar with most NT/U21 players to make the selection. In this case, the NT manager can just:
1. Do nothing and not nominate anyone.
2. Discuss on who to nominate with the local community.
3. Randomly select someone from the list for the sake of nominating someone. This might be meaningless to him and many others, but I reckon anyone in the top 5 (after so many seasons) would more or less deserve it.