I think more units should be granted to the garrisons by the immigrant chains, nothing outrageous, just +1 native unit for every level. For example, if you conquer Beneventum, the immigrant chain there will grant a garrison of 1 Italic Spearmen, level 2 will add 1 Italic Swordsmen, level 3 will give 2 Campanian Hoplites, and the final level will have you at a grand total of 2 Campanian Hoplites and Campanian Cavalry.

Building the Immigrant building in Taras would add Taras' Area of Recruitment units to the garrison, and loads of other examples. Some garrison additions will be stronger than the others, but that would be inevitable and part of the fun of not knowing what you'll get and being pleasantly surprised by the being able to recruit approximations of other faction's rosters into your empire (to an extent).