I've thought about this idea before as well, it's a fun idea and it would allow coaches to customize their programs nicely. Benefits could be modest, but noteworthy. Division 1 teams might have 3 assistant slots, division 2 might have 2 assistants, D3 teams could have one assistant. Shagnew had some good ideas, here are a few more:
1) the scout - allows the coach to specify the attributes that are scouted on a scouting trip, or alternatively could give a slight discount to FSS costs.
2) the recruiter - gives a slight bonus to the value of recruiting actions
3) the offensive guru - gives a moderate bonus to offensive set practice/IQ growth.
4) the defensive guru - gives moderate bonus to defensive set practice/IQ growth.
5) the fundamentals guru - gives a slight bonus to individual practice/attribute growth.
6) the gameplanner - gives you an extra checkbox for adjustments on the gameplanning page?
7) the sleazebucket - gives booster gifts a greater chance of being accepted, or not getting caught ; )
It could be as simple as making your 1-2-3 choices of assistants and being done with it, or assistants could have varying attributes and there might be some sort of competitive bidding/hiring process(more complicated, obviously) . Loyalty could take a hit for switching assistants too often.