Variety is great.
Consistency is better. Re-open thread.
Seriously, if you're making a chapter and want it to stand out, it has to have something that binds it, and theme and difficulty curve are tools to that end, otherwise you're not making a chapter, but a collection of disparate levels, and then collecting them in a set of 20 is purely arbitrary.
Do you want your chapter to be a concept album or a greatest hits? What do people revere the most? Sgt. Pepper's Lonely Heart's Club Band
, or The Beatles Collection
? Ziggy Stardust
or Best of Bowie
You can use different tilesets and still keep a theme. Your chapter could be themed by design, using primarilly missile based challenges for example, but it does need a binder or it fails on an artistic level.