In addition to being a learner class it's also designed as a side-kick class or for drop-in players.
Both the Battlemaster and Champion fall behind other classes - particularly spellcasting classes - at levels 4 and 5. Everyone gets a feat at level 4 but lots of other classes get other things too: the Eldritch Knight gets an extra spell, for example. The combat classes generally get a second attack at level 5, but the Paladin and Ranger get two second level spells and the Barbarian gets Fast Movement. The Battlemaster and Champion get nothing extra at either level.
I suggest bringing forward Indomitable and allowing the Battlemaster and Champion to use their Proficiency Bonus instead of their Dex modifier when calculating AC (and later Initiative). Which way round you have them I leave to you.
I also suggest dropping the 6th level feat and replacing it with a Fighting Style or Action Surge for the Champion and 1 free maneuver per round for the Battlemaster.