Possible system
1: Each familly of monster has a Gauge to fill, as you kill monsters from X-familly "Must gain SP in the process" the gague will fill depending on the ammount of SP you gain. When the gague is full you can extract a materia from your bestiary or continue to stockpile points for better chances at HQ materia.
2: Gaining % completion in the bestiary rewards with a set type of materia befitting the work needed to reach such completion. IE: 10% gives you a +1 base state materia 50% gives +8 base stat. Could be any number of combinations.
3: Finishing a certain monster familly in the besitary rewards with a special materia that adds a damage bonus to ____familly of monsters. IE: 15% damage boost to Forgekin ext.
I think the materia system and bestiary combined would create a much needed time sink into the game.
These are all just suggestions within the thread and not a single cohesive plan, I just wish to give as much suggestions in the top post to get the topic flowing for the developers to read