I'm currently trying to rebalance certain aspects of the D&D 5e rule set. The overall goal is to open up options for my players so they have a more diverse pool of features to choose from. I don't want them to feel like they "have" to or "can't" pick a certain option, because of it being just that strong or that weak compared to alternatives.
Currently I'm looking at the Warlock invocations; my analysis shows me that a few of them are worse than others:
Thief of Five Fates
Sign of Ill Omen
Minions of Chaos
What most of these have in common is that they give warlocks access to a spell they otherwise wouldn't once per long rest, while still using one of their spell slots. Compare this to invocations such as Mask of Many Faces (Disguise Self at will) and Devil's Sight (120ft super-darkvision).
The utility of these invocations is very lacking. Warlocks don't have a problem with the diversity of their spells; the problem is with having available spell slots. I'm trying to determine whether removing the spell slot cost from these invocations and making them long rest abilities instead would balance them, or make them a more attractive option rather than a "don't bother" option.
In the context of re-balancing the Warlock invocations, would making the aformentioned invocations long rest abilities, without costing a spell slot, make them balanced compared to the other available options? Or is it too much/not enough of a change?
If anyone thinks the invocations I mentioned are fine as they are, I would love to hear your reasoning as well, in case I missed a core point!
I'm asking if this change would be balanced in a non-homebrewed version of 5e. To clarify, my group generally has about two short rests per adventuring day.