As you know, the monks are absolutely out of meta. Although they would be buffed, they would remain being useless. So i propose some ideas for rework them.
For example in AoE2, monks although healers they had others choers, as taking relics or convert units. I propose for european civs a new mechanic for which monks could been used to generate experience.
My idea is that the monks could been used to pray in church (similar to greek villagers in AoM) and in native settlements. So when the monks pray the player would earn experience.
This new mechanic too could be useful to give the native settlements a new utility and it would simulate the tendency of the colinizers to spread their religion to the lands they reached.
I have this idea, if you have more propose it too. Which is sure iss that europeans monks must be reworked. If the revolutions was reworked when they was useless, monks could be reworked too and so the game would have a new interesant mechanic that give the gameplay more dynamism.