Contrary to what Hera said, there are some older unique units that lack elite upgrades. First is missionaries (basically like monks, which are a generic unit without elite upgrades, but with so many monk techs, I think monks/missionaries are fine). Second is Condos (an imperial age unit, like the mounted trebuchet - no problem here due to being locked behind imp). Third are slingers - an anti-infantry unit that is available in castle age and retains its anti-infantry role in imp. I don’t recall hearing complaints about slingers, and they’ve been this way for a while. Fourth is warrior priests, who have good utility in castle age, and benefit from a UT in imperial age. They’re good in castle age, but their main use is as a utility unit who can pick up relics and heal.
I think that Jian swordsmen could be improved by exchanging some of its gold cost for food cost. That would make it more expensive in castle age (when food is more valuable than gold), but make it more spammable in imperial age (where it falls off outside of its anti-archer role). Ideally, Jian swordsmen would be like slingers, maintaining a strong anti-archer role in imperial age, but their gold cost is pretty high. And unlike slingers, they don’t have the survivability advantage that comes from being a ranged unit.
Grenadiers benefit so much from thunderclap bombs that it almost acts as an elite upgrade (except an actual elite upgrade would improve survivability). Probably fine as is. If they did get an elite upgrade, it would basically need to be focused solely on survivability (probably something like +5 or 10 hp, and maybe a point of armor)
War chariots also benefit from a UT, but that is far less impactful (and an Elite upgrade would improve war chariot survivability). Probably the unit here that most needs an elite upgrade, since it has hard counters in castle age that just become harder in imp.
Xianbei raiders have parthian tactics, the melee armor UT, and an scaling hp bonus that all combine to make an elite upgrade less necessary (and Cao Cao’s aura can improve their attack speed). An elite upgrade might help them scale better, but I think it would mainly allow them getting weakened in castle age, and the upgrade would frankly need to be pretty insignificant.
But between the elite techs and the precedence of slingers, I think the only one that really needs an elite upgrade is the War Chariot. Jian swordsmen would also be fairly easy to balance through an elite upgrade, but I think they can be balanced without it.