Aircraft basically live in the sky. Which makes the sky very important, especially in a flight simulator. I think eventually, we need to see all the possible cloud formations that are true to life. As mentioned in a previous Q&A, they (MSFS) are striving to create a replica of the earth.
Below is an image that illustrates some of the cloud formations that we see in our everyday-to-day life.
Source: List of cloud types - Wikipedia
This image is only the tip of the iceberg regarding clouds, as there is much more to it, however, it would be amazing to have these implemented into the simulator, especially to match the live weather!
There are multiple images online such as the ones below that really make the sky so beautiful to look at, and I’m sure these clouds would make massive improvements to the flying experience. 
Source: https://twitter.com/nickywhiffin/status/1324671488578985986?s=20
Source:Cool Clouds Photograph by SJ Lindahl | Pixels
Source: The Coolest Weather Picture Ever! | A Word of Weather
Source: https://twitter.com/mark_tarello/status/859235641699250177?s=20
Not forgetting the noctilucent cloud which is one of my favourites!
Source: Noctilucent clouds - Met Office
And did anyone ask for some arcus clouds? I’d hate to be under one…
Source: Arcus cloud - Wikipedia