It would make their already very powerfull early imp even more powerfull.
I think with HCA people still go hussar for mobility and because the upgrades are free, but some halbs to complement jannisaries and BBC would definetly be a thing.
They would become quite good in trash wars since skirms aren’t as important as the other 2 there and now they will have acces to both.
Making a techswitch to counter cavalry becomes way easier cause teching into halb is way faster than camels.
The ability to counter cavalry in and of itself isn’t much of a gamechanger though, they already have good ranged units to kite heavy cavalry and grind them down and acces to camels.
They can finally deal with massed post imp elephants.
Overall it would just cover a lot of the civs weaknesses, makes them strong at later stages of the game were they weren’t strong before and make them strong at times in the game where they already are strong in the first place.
In other words it would be a bad idea.
If they get halb they also get pike but that doesn’t even have half the impact of halb.
It would just make being denied gold in castle hurt much less and it adds a good complementary unit for certain army compositions.
They can get pikes and the impact wouldn’t actually be that big, pikes just aren’t that impactfull of a unit.