Thanks @
Yunru for making me think about Haste. There are a number of bonus action attacks and the like that trigger off an Attack action, so aren't usable with GFB or BB. But Haste explicitly allows the Attack action, just limitting it to a single attack. So You can use one of the attack cantrips and then an Attack action from Haste which then triggers a bonus action attack (dual wielding, polearm), ranger Hordebreaker extra attack, UA Ranger Beastmaster beast reaction attack, or whatever.
(Didn't list crossbow expert because we're talking GWF/BB which are melee, and Sage advice nerfed melee+hand crossbow.)
Not needed with quickened GFB/BB, but if you aren't a sorcerer or want to conserve SP still useful.