In particular, go for rules that announcers typically screw up, or rules that are notably different from NFL to NCAA to NFHS. I'd go with:
- "Down by contact" is big lately, tied into "the ground can't cause a fumble".
- Pass interference (with the bit in the Steelers game, it's a hot topic)
- For that matter, catch/no catch. This one, in particular, needs lots of film to illustrate your point - although you could also spend a decent amount of time discussing how things change in the end zone, as well as the force-out rule.
- Touchbacks - this will mostly be about the rule differences, and probably a short segment at that.
- Forward progress - a simple enough rule, but there will still be people who want to go by location of feet, especially on a sideline play. Not sure there's enough for a whole show here.
- Penalty spots - I'd guess there are a fair number of people who are confused about when the penalty is assessed from the end of the play as opposed to previous spot or spot of the foul.
I'm curious, how long a show will this be? I'm presuming it'll be on a public access station, which means no (or at least few) commercials. Half an hour actually becomes a fairly long time to fill.
You'll definitely want a bunch of film clips. The rights to any NFL or NCAA game are going to be very tightly controlled - you *might* get permission, but I'm guessing not (although I've never tried) - so hopefully you have or can get ahold of a decently sized library of H.S. tapes.
If the cable thing doesn't work out, you could always produce the segments yourself and publish them as podcasts, which are currently The Next Big Thing.
Good luck!