Introduce a new building (or use an existing like storehouse or command post) to offer trading of NTGs.
Take the original Marco option of trading in 3 of one type for 2 of another but make it a much wider scope facility so that there is always an opportunity to trade for the type you need.
It will always be really tough to make all nations equally appealing at every age.
It will probably spoil multiplayer to open up all Vs all attacking (although I do think MP battle rewards are nonsensical right now). You could open up attacking to all but you need to incentivise players to want to hit high, and the incentive isn't big enough now - mainly because of NTG requirements. There's also the manipulation of the new event system to consider, if high end players can hit baby bases it might lead to these bases getting hunted.
Having a comprehensive NTG trading post feels like the way to go.
Please excuse me while I return to hitting Next until I see porcelain...