I'd like there to be a good reason to keep tier 1-2 units around in the later parts of the game.
One way to make them relevant would be to give them an upkeep reduction when inside cities/outposts that goes past the minimum upkeep units normally have, down to 1 or 2 gold/mana per turn perhaps.
Maybe have it tied to town hall upgrades, and maybe have it as an upgrade to an outpost. The highest tier town hall could make it affect tier 3, perhaps?
It's realistic, it makes sense, and it would give more variety to late game.