Cleave's second target has to be within your reach, and within 5' of the original target. So a reach weapon with Cleave (such as the halberd) could hit a second target that's "behind" the first target.
Flex only increases the damage die size, so only increases average damage by 1. However it allows you to do so when using a shield (so a higher AC), as well as take advantage of the Dueling fighting style for another +2 damage. And if you use the Dual Wielder feat, you could have two 1d10 longswords as weapons.
Graze: Poisons can take effect on damage:
Dungeon Master's Guide said:
(Type: Injury) A creature that takes slashing or piercing damage from a weapon or piece of ammunition coated with injury poison is exposed to its effects.
So a poisoned glaive, doing slashing damage with the Graze weapon mastery, would
always inflict its poison effect. This could be Drow poison, giving the poisoned condition or knocking unconscious, or serpent or wyvern poison, doing additional damage along the way.
If there are other effects that take effect on damage rather than on hit, they could also be used.
Sap: A mace, morningstar, or flail becomes a very useful defensive weapon. It gives a useful mechanical support to the trope of the cleric or paladin using a non-bladed weapon like a mace or morningstar, likely combined with a shield, as it dramatically reduces the chance of being hit. They'd need to take the feat for that, though. (I'll be a little surprised if paladin doesn't get a weapon mastery trait for at least 1 weapon.)
Or just a fighter built more to be a tank could do the same. Add the Sentinel feat to stop enemies trying to move past you, and their only option is to attack you with disadvantage.
Slow: Clearly useful for ranged weapons, to keep opponents at range longer. However it's also available on the whip, which is a reach weapon, or could be applied to any other reach weapon at higher fighter levels.
It would be interesting to see an Eldritch Knight combo this with Ray of Frost, as the two speed reductions should stack for a total -20 move speed. Feels like something to use as a kiting strategy.
Alternatively, a fighter using a heavy weapon (maybe even with reach) with two properties, uses Slow on his first attack and then Push on the last. Would be nice to combo with Polearm Master, since that allows repeated use of the opportunity attack when opponents enter your range (plus Sentinel to lock their position?).
For NPCs, a thug with a club could hit an opponent as a way of getting a head start for running away.
Vex: This is one that feels like it would work very well with the rogue, but the rogue probably isn't getting weapon mastery. If it does, I would bet on it only getting mastery over one weapon, and picking an offhand weapon with Nick seems the more reasonable choice. If the rogue gets two weapons (or gets 1 plus the feat), shortsword + dagger for Vex + Nick seems the natural combo, allowing Sneak Attack on the dagger's attack while still having Cunning Action available, even in a solo fight with no hiding.
Aside from that, almost all qualifying weapons (ammunition, finesse, light) that could have Vex already do, with those that don't having a ready alternative, so I don't see much likelihood of using the fighter's mastery swapping for this, except maybe on the whip — a finesse weapon with reach with almost constant advantage. Not sure there's value with that compared to, say, a rapier, though, which is more damage but without the reach.