Yeah, Soulbound Weapons definitely fit. One idea that occurred to me was leveling up by enemy reclassification. In other words, enemies that were once treated as Individuals may be reclassified as Squad units following certain upgrades or level-ups. Squads, in turn, may be reclassified as Swarms when you get even stronger. So for example, in the beginning of the game, Knight of Vermach would be treated as Individuals units, so being surrounded by a dozen of them could get you killed. However, if you acquired your Soulbound Weapon, those same soldiers would be treated as Squad units, so a dozen of them would be like having only 3 or 4 enemies, much more manageable.

For summons, I'm toying with the idea that some (though not all) of them would come from the same world as the PCs. So you could occasionally encounter species in the wild that you're capable of summoning. I'm not keen on playing Pokemon, so I don't want to emphasize this part, but I do think it could be fun to discover creatures you'd like to tame and recruit. In fact, some of these monsters could be sentient enough to have their own factions.

What do you think?