I know a lot of people are wanting other feats, cantrips and spells, especially people who want to play Eldritch Knights. The problem with things added after the PHB is that sometimes you run into perfectly legal feats, spells or what have you, that are simply stronger than what the PHB offers at the equivalent level. So, if they do bring in anything from outside, (which I would encourage) they'd need to rebalance them to make sure they're on par with phb abilities.

As a GM I don't know how many times I've seen official splatbooks have abilities, feats or spells that a GM just has to say "no, that's broken. I'm not allowing it."