OUTERRA:
Hi, I found this intriguing program, in this link: http://www.outerra.com/
WOW! It's so realistic! Not only terrain like Space Engine, but volumetric clouds, more realistic water with waves, vegetation and ambient sound, and it's still the demo! A shame it only focus on Earth!
Imagine if Space Engine had all those features! Well, Space Engine already have the advantage that allows you to explore the universe. But think of adding all those things too! Imagine riding a real car or flying using real airplane in an alien planet, like on TRAPPIST-1 planets!
Combine the two and Space Engine will turn to be the best simulator EVER!