Yeah, it's definitely hard, but you need to spend time remembering what made the original good.
It can be done. Civ2 outpaced Civ. Jagged Alliance 2 outpaced Jagged Alliance. Warcraft 2 outpaced Warcraft.
Granted, these were different genres, but they took what made the first games great and enhanced it. They added rather than subtracted, always keeping the "fun factor" of the original in mind.
Sequels that remove key/defining features rarely work, like Red Baron 2, which removed things such as volumetric clouds. It was rushed out, buggy, and lacking. Instant failure. Hmm, sounds familiar.