I came here by way of a forum search. I was going to suggest a "Luxury goods" expansion to the game myself. More specifically, I think it makes more sense, in terms of the games mechanics and in terms of [somewhat] reflecting development in the real world. It seems like the only "late-game"/"upper-tier" thing to do is to stockpile money and then go to war (this excludes the metagame; the meta politics are a lot of fun to be a part of). Low-tier nations focus on growth and developmental or military resources. A third category of resources (like luxury) could substantially alter the upper-tier style of gameplay if you limit their utility to nations above a certain infrastructure threshold; either a hard limit (for example, 25k+) or have access to luxuries limited the same way all Projects are limited.
In either case, access to luxuries could be obtained only after unlocking a special Project, granting your nation access to 'bonus' production facilities that utilize goods already in the game. For example, oil could give access to plastics, a new resource demanded by populations in nations of 25k+ infrastructure whose happiness now impacts productivity -- for the trade off that their can boost happiness beyond 100%, giving the upper-tier players with this project and/or resources the ability to further boost income beyond the 115% commerce limit.
The reason they should incorporate resources in the game is that it gives the low-tier players yet another opportunity to grow by facilitating and increase in demand for resources used for more purposes.