This is not a suggestions question.
I'm not looking for someone to suggest how I should balance ability X. I want to know the "rules" or customs for balancing. I am also not looking for rules on class/class-option creation, only ways to create replacements for certain class-specialization abilities based on level.
With that in mind, carry on.
I am thinking of trying to home-brew an alternate Pact of the Old One 14th Level Pact Boon which is not Create Thrall. But before I present it to my DM, I want to make sure it's balanced. And this is where I'm having difficulty, because I have no context within which to balance a 14th-level Pact Boon which is on-par with the others (Dark Delirium, Create Thrall,...) while not broken.
Is there any guidance on creating abilities like this? I understand the DMG has guidance on creating spells, but I'm not sure what level spell a Pact Ability (which is effectively the same as an 18th-level feature for a Fighter) would be most similar to, or even if it would be similar at all, since abilities often do things that spells can't, or beyond the power of a spell.
Is there any official guidance on creating class options? If there is no official guidance, are there any tested DM conventions?
For an answer to be "good", it should include something besides "Ask your GM", because I want a balanced plan before presenting the idea to the GM. Also, the GM is new (to being a GM, not new to the system), so I don't want to overload them with research.