Even if Hand Cannoneers are buffed further, they still will be hard to justify using because they are NOT a trash unit, and are ONLY available in the Imperial Age. The Archer line are not only better in the anti-infantry role than Hand Cannoneers are, but by the time you reach Imperial Age, you have likely been already massing up a leftover army of Archers and Crossbowmen from the Feudal Age and Castle Age mid-game period, and then finally turning them into Arbalesters.
Now…I am NOT advocating for Hand Cannoneers to replace the Arbalesters. But what I am staying is that the Hand Cannoneer should be buffed so that civs that LACK the Arbalester upgrade and/or lack Bracer, should have the Hand Cannoneer as a handy option if that civ needs to create anti-infantry forces.
Teutons definitely benefit from this, because that civ gets Ring Archer Armor, but no Bracer or Arbalest upgrade. And unless as a Teuton player, you are willing to make mediocre Bodkin Arrow Crossbowmen, you would be wiser going with the Hand Cannoneer, to be a ranged anti-infantry option.
What if you are facing a horde of Gothic Huskarls? Crossbows and Arbalesters fail miserable against Huskarls…but Hand Cannoneers do well. Not even Heavy Scorpions can defeat Huskarls easily.
Franks, while yes, their Throwing Axemen are a god counter to enemy infantry, you could be in a situation where all your castles are destroyed, and you cannot build any new ones. But Archery Ranges are cheaper to build, and faster to mass up. So as Franks, Hand Cannoneers are a good option, as again, that civ has no Arbalester nor Bracer.
The Burgundians in particular can make good use of Hand Cannoneers due to their gunpowder bonus. So if you are Burgundians and are massing Paladins/Hussars but are facing enemy Halberdiers, you can train Hand Cannoneers to protect your cavalry from those nasty pointy weapons (not to mention, also against enemy Burgundian players who create Flemish Militia).
Persians, as @francisco2002 has mentioned, used to go with Hand Cannoneers in their strategy due to the fact that Persians lack Bracer and Arbalester. Only now, with the Trash Crossbowmen unique tech, the Persians are better off ignoring Hand Cannoneers and just sticking with the wood-cost archers…despite not being the most upgrade of archers.