Oh another thing I would love to see is an invocation that lets your casket serve as a vessel for souls sort of like in dark souls and hollow knight

Maybe as a reaction when a creature within a certain range you dies you can siphon magic energy from its soul into your casket. These souls remain for up to 20 minutes before their power dissipates. The strength of the stored souls is equal to the total cr level of creatures harvested using your vessel.

You can store up to your warlock level in soul strength. As an action you can focus all of your your stored souls to regain Xd4 where X is the total cr of souls in your vessel and destroying the souls in the process.

Alternatively, you can send souls with cr greater than your warlock spell slot level to deepen your connection to your patron.