I've started running Curse of Strahd and have been having a ton of fun making wacky homebrew things for my party and tweaking the module. Unbeknownst to the players so far, I have made them all a secret Dark Power trying to use them to usurp Strahd and Vampyr, and they're just waiting to offer their dark gift.
For example, I have a druid with some fire giant ancestry who loves Produce Flame and is their front line, so he'll be offered a slightly weaker Rage with the Path of the Storm Herald desert feature to use once per long rest. The celestial warlock would get charges to boost his Eldritch Blast damage and some secondary healing in exchange for sapping his life.
Giving my players powerful upgrades alongside typical class advancement can make some encounters harder to balance around, but as long as we're all having fun who cares about balance??
Three years ago I wrote some notes regarding the lore and background of the world I placed my players in. Now they're invested and actually hunting down the villain I added only as a background.
Nothing is as good as players being invested and exploring the world well past my initial ideas.
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