- Upgrades required to access higher tier resources. Ie. Forest initially only has wood, needs an upgrade to access magic wood; mountains initially only access stone, need an upgrade to access iron, etc. this would probably mean bringing the core resource regions closer to the plains?
Or you need better dungeon maps, and/or have to travel farther/deeper into more dangerous sections of those terrain types. You want the really good stuff, that is 2-3 mountain ranges away, and has quite a bit of travel time, and monsters require more gear/levels to fight safely. Rarity could play a larger role later, having a Common Uncommon pool.
A lot of the character strength already comes from items and class skills. So it isn't impossible to see having to grind out one tier of gear to get to grinding the next rarity zone's level.