For realism i would add some sun rays for one part. The color pallet needs to be edited maybe you should change the color of the terrain to a more natural color (The sand seems saturated) like the sand and the grass. make sure that the colors blend together well and look at some cinematic picture or reference a picture to get the colors slightly and make it look good on roblox.
aesthetically you could add in like rocks and trees maybe particles for the water for like gizers or soothing that would add to the look of the hole scene or even some mountains or caves.
i do not see shadows but this is probably on but you should have shadow map on and the diffuse and spectacular sliders in lighting for metal if you want metal anywhere but it also effects lighting so i would mess with that also.
it all depends on the style of look your looking for.
i would do more pictures to help out determine what the hole scene is though.
The lighting part of this place, and the terrain looks amazing for a pretty good start. Overall I would like to give some suggestions that could improve your terrain.
As for realism, it is important to experiment with different effects like sun rays, etc. And to look to work with adding particle effects to your terrain this would really bring your scene together as well, you could try doing some effect tests, to see which one adds a little bit of additional realism. You could get some inspiration, by looking at some examples on how you want your terrain to come together/or by looking at different designs.
As well it would be better if you add minor vegetation objects around the terrain and water, such as plants, trees, rocks, leaves, lily pads, cattails, and more for. The water that would add a little realistic environment to the whole scene or even some hills or instead of the terrain being all flat.
But for the most part I would fill up the terrain with vegetation and different parts of the scene with mountains, Pedestrian Trail Bridge, and other minor things you could possibly add.
But it really depends if you just want to add just vegetation to your scene instead of adding all the other details you could get some ideas from (Goldencowboy) portfolio and some pictures online i do believe you could find some ideas from those.