I would say reduce cost of Militia line at Brewery, not eliminate one or the other resource cost. Either that, or remove the food cost while increasing the gold slightly, for balance, say by 5? (I know, who wants to pay more gold… eh, whatever.)
For each of these buildings, you should be able to build as many as you want, but some should only grant the effect once, no stacking things like resource savings or training time reduction.
For Franks - Tourney Field: +1 Attack and +1 LOS to Knight line. Requires a Castle and a Market, and to have researched Chivalry in order to build. Costs 300 wood and 100 stone to build, and costs a slow trickle of gold (equal to a relic, so they would cancel each other out) and food (5 for every gold) in order to keep up the effect, which can be manually turned off by the owner. Effect does not stack. I know the Franks have a great Paladin, but this would really bring out the feeling of the chivalric age very well, and the effect can be lost without even losing the building because of the gold cost. Also, having it would force a confrontation.
For Chinese, what about instead giving them - Palace: Grants ability to increase max pop by 20. Imperial age, requires a castle, plus all the houses needed to bring up the population space to the new max. Costs 250 wood, 100 gold, and 75 stone.
Italians - Trade workshop: Yeah, the one from the scenario editor. This one would effectively be a relic, but with the catch that it can be converted by any player just by having the most units nearby (like converting torches, or the monument in King of the Hill). Max 5 stacks of the effect to any player. Costs 250 wood.
Aztecs, Incas, and Mayans - Temple: Sacrifice 10 villagers to increase Militia line stats. First 10, +.5 speed. Second 10, +7 health. Third 10, +1 Attack. Requires a Monastery, and 1 garrisoned monk to keep the effect up. Costs 300 stone.
Saracens - Shisha lounge: Garrison 5 villagers to get 15% discount on food and wood costs at University and 5% on gold costs at Market. Requires a University and a Market. Effect does not stack. Costs 200 wood.
What do you think? Honestly, some of these are probably wildly unbalanced, but I thought they were colorful.