D&D 5E Idea: Fighters can use any fighter manuever with reduced die

[MortalPlague]

Actually, divine casters did (in 3.x & classic D&D, and, it seems in 5e so far), in effect, 'know' every spell on their list. Once they get to the top spell level, that's it, they're all "the same" in that sense. Wizards have a list of known spells, but it can be pretty large - much larger than the list of 'obvious best' and outright broken spells that you'd be stupid not to use.

I mis-spoke. What I mean is that the casters couldn't have access to every spell at a given time at 30th level. Fighters shouldn't either.

Though retraining maneuvers could be an option...

 

[Minigiant]

I think fighter should be able to at least attempt unlearned maneuvers.

Combat Improvisation
Your mastery of the combat arts allows you to try maneuvers you have not mastered yet.
Benefit: You may attempt a maneuver from the fighter maneuver list that you have not learned. You may only spend one die on the maneuver this way and the spend die is treated as a 1d4 instead of its original value for the purposes of the maneuver.

 

[Crowsion]

What if we had a point buy system. Where at every level you gain a number of points that you use to "upgrade" your maneuvers. This mimics your training, and eventual mastery of specific maneuvers. At first level, you might start off with a couple extra points, to make your initial allocation. To get the actual progression right, might take a little bit of work but it could work, whenever you use that particular maneuver you gain the bonus based on your level of mastery. At higher levels you can add other transitional power increases such as being able to be used twice per round, but generally you can use two maneuvers per round, one that you use with an action, and one that you use with a reaction.

Level Expertise points Maximum Investment
1 +4 2
2 +2 2
3 +2 3
4 +2 3
5 +2 4
6 +2 4
7 +2 5
Bare with me, I don't know how to set up a properly formatted table yet, sorry.
As an example, take deadly strike,

Points Additional Damage
1 +1d4
2 +1d6
3 +1d6
4 +1d8
5 +1d8
6 +2d6
7 usable 2/round