AoE2 DE Mercenaries Mod

I remember making a "mercenaries" mod in HD. I kinda did it on a whim with little understanding of meta. I'm wondering what would be the best units/techs/mechanics to supplement civs.

For example: Giving Turks access to a quasi-trash Pike that may cost gold but is worth its price in the gold it costs, etc... or units that require relics.

Any ideas as to what would be a good idea for this?