How do you balance creative expression with brand guidelines while designing?

Just keep some commonalities. You can be more expressive on your website than in the app. The app has to be more strict and change its design less frequent, because it's a lot harder to change. the website can change every other month.

if you can't design creative landing page with your brand guides, that means your brand guides are outdated or just bad.

Design is a language of creativity itself. So, how to express an idea through design depends upon the designers themselves - which makes each and every design unique! As for design guidelines, I personally like simplicity and focus only on a few elements while designing like - what's the most attractive element in the project, or how would the users respond to the (specific) element... Flexibility, as a part of design, is just like storing water in different containers. No matter where you store it, the water can change the guise of the containers ^^

This is a tough question. Guidelines are like a playing field that you have to be creative inside.