on the topic of building material, it'd be interesting if we could use it to make mansion improvements. like aqua's garden for food stuff consumables for temp stat bonuses or EXP gain, a training room for further improving darkness' HP and passive protection, and maybe a small +5 total strength boost. maybe with the option of training everyone else individually too. could also be an option for group training that applies to all party members. a room for yoga, meditation, etc. for MP and willpower improvements. a workshop for any of kazuma's bigger projects he wants to sell off. or maybe to build a car, boat, airship for party use.
maybe even using some to build camping equipment that the player can use to rest, down consumable, etc when an area is clear of enemies. could be an idea to have the camping equipment to be upgradable from consumable equipment to be infinitely reusable (after packing it all up again), provide minor temp stat bonuses, etc.
of course if all of the ideas are used we'd need to have building materials be a purchasable item to a limited daily extent at the least.