I would love some limit to Long Rest spams, but only on higher difficulties perhaps, something like DOS2 Tactician mode.
As to how to implement it - progressing quests would be one way to do it, at least some impactful ones, like the one in the Druid Grove.
Alternatively, perhaps make taking a long rest drain XP? Whenever you long rest you need to pay Y XP to take it. Effectively limiting progression if you spam. Never de-level a PC though.
On that note, I do feel like more short rests should be allowed, 2-3 per day. Maybe increase the number of short rests the party takes throughout the campaign. For levels 1-4 one short rest is fine, but later warlocks will really fall behind full casters in the amount of spells they can cast.
