I think Cores should be split into 2 resources, Infusion Cores and Masterwork Cores. Infusion Cores can be relatively common, maybe earned from strike completions, adventures, raids, Crucible, etc.. A basic material for a basic purpose, earned through pretty basic gameplay. Meanwhile Masterwork cores are rarer and can only be obtained through dismantling legendaries level 4 or higher and maybe from dismantling Exotics as well. Bungie has said they want some form of resource for Infusion, and I think this could work.