There are feats to allow players to gain proficiency in I believe all saved by Charisma, since you get two by default, you only need 4 more, which arguably takes all your feats, but if you want to go that route it is available. You could certainly grant additional proficiencies as a player levels up though. I don't see that it would be terribly unbalancing if classes gained an additional proficiency every 5 levels or so. That would cover all 4 by 20th level.
Also keep in mind that the proficiency progression is fairly slow, it's only +6 at 20th, so w/o magical buffs, the BEST DC you could have is only 19 (8+6+5).