Why not allow crafters to craft an item that allows Rare and Relic weapons/equipment to allow materia to meld on to them. The craft would be 60-70% success rate at best. It should use high level gathering materials, low-mid level dungeon items and crafted weapons/gear. An example would be for the item to add materia on an Allagan Cane, you would need:

30x Gold Ore
1-3x Unique dungeon drops (something like the Bloody Bardiche Head in Wanderers Palace that are unique to that dungeon but in lower levels)
1x Alkalurops
10-20x Clusters
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Healing Crystal

Then you would take the item you just crafted and bring it to Gerolt along with some philo items. If your crafted item (healing crystal) is HQ then your Allagan Cane would have two materia slots, if it was normal then you would have one slot (the slots that grant a 100% success rate). For the higher weapons from the new patch you would need the new gathering items, higher level dungeon drops, the new higher level crafting gear and the new myth items to create the materia enhanced i10x items. You can give the new items a low materia cap to make it valuable to have but not an overwhelming must have.

This way you allow gatherers, crafters and lower leveled players to be able to get some money. Philo tomes will still be valuable when the new tomes come out, crafted items will be wanted and used in low and high end games and materia will be useful even when you are decked out in full allagan gear.