*blows dust off this thread*
So, at the risk of adding way too many resources to the mod, I nevertheless want to open discussion about two additional resources that might be useful:
1. Timber
I think someone brought this up before but I resisted it at the time, but now I have been thinking about it some more and I think it has several advantages. My first idea is to give it a significant (such as 50%) production bonus for all ships, maybe accounting for it in overall ship costs, but building ships with Timber should be faster than it is now without. Major Ocean going vessels (Galleons, Frigates, Ships of the Line, Ironclads) could even require Timber outright. Then we could place Timber in locations with historically significant locations, for example Lebanon cedar to benefit Phoenicia. In addition, Timber is worthwhile as a resource that benefits northern climates, which are not already too resource dense. I especially thought about this in the context of settling North America, for which there are often not too many incentives. Historically timber was an important product of North American colonies, especially for the English.
2. Nitre/Saltpetre
I think this is interesting in two contexts: as a component of gunpowder and as a component of fertiliser. Given the historical distribution of saltpetre I don't think it makes sense to add it as a required resource to all gunpowder units because it would only ahistorically lock most civs out of them. However there are other ways to link them. For example there could be a Renaissance era building that gives additional experience to Gunpowder and Siege units but requires Saltpetre (Castles expire at the time and Gunpowder unit experience could be removed from Barracks). Later on, I would like to reform the unit resource requirement rules entirely and lock units less to resources. Instead having a number of resources would give free upkeep for a number of units that rely on the resource, and having Saltpetre in the game for that purpose would round out gunpowder units in that purpose. Regarding fertiliser, instead of giving +1 food to Farms with Biology the tech could unlock a new "Modern Farm" improvement that requires Saltpeter to be built. In that sense it would also provide more stuff to do for workers.
I think both of those don't really compete with all the luxury resources that have been added so far (except maybe in terms of space) and instead fill interesting niches for strategic resources of the mid game with late game applications. Right now I only want to open discussion of them, and if someone wants to suggest or research plausible locations for them that would also help a lot in making the decision. It's not that easy to find out beyond famous sources like Saltpetre in Chile and India.