Make Everything Modern
Make everything Modern! I'm tired of having to use some Classic features on my Modern Team and Communication sites. Give me a Modern calendar, task/issues list, settings pages, permissions, etc. I don't want to see any Classic UI on my sites and I don't want to switch back and forth between Modern and Classic to accomplish tasks. I'm trying to get my company to fully jump in on Modern but having to explain why some things are still Classic makes it difficult and confuses users. They become accustomed to the Modern UI, and then are surprised and confused when presented with a Classic UI. It's like a separate site and doesn't belong together.
Tom Burke commented
For Classic sites that are coming into Modern via the automatic changes, we need better synergy between the experiences. A *big* part of this is what Tyler mentions above about switching into Classic mode when hitting system pages, and conversely switching to Modern when hitting Library and List pages. It's disjointed and off-putting.