So, shooting myself in the foot here because I just got a shield myself, but. Corrin (the Black Arrow) and I have been discussing ideas about shield mechanics for awhile, and Skip sounded interested when we first mentioned them but I completely forgot to post these thoughts. So, I figure I will post now!
Due to the rules of LARP, shields are really strong. They stop almost every attack (except dragons and siege weapons) and you cannot just wail on them to knock the person wielding the shield down because that is too aggressive for our game. As such, the only immediate counters to a shield are to have a shield yourself, or to have a numerical advantage in order to use flanking. This makes shields hard to beat mechanically, especially for LARPers who lack the funds to just go out and buy their own shields.
Corrin and I had the following idea to try and rebalance shield mechanics and also promote some class variety in game: give mages (and clerics) the ability to enchant a weapon to "break" the next shield it strikes. This would give another counter to shields, give mages and clerics more combat utility, and be limited enough that it would not obsolete shields.