Hello, my name is Honeydewloon and I am a mesh maker and texture artist on ROBLOX. My current pipeline is 3Ds Max or Blender → Zbrush → Blender for retopology → Substance Painter for Texturing.
However, knowing that the majority of people on ROBLOX use Blender 3D for their modeling ventures, I’d like to introduce a few tricks to get the realism or beauty of your mesh up.
1. "Static Normals"
If you have the knowledge to use normal maps, USE THEM! assuming that your model is a static model, like a tree, rock, or otherwise un-moving object, you can bake your normal maps into the diffuse of your mesh, creating a hardwired lighting effect that makes it look high poly.
What I personally do is: I mimic the lighting of my ROBLOX game in Blender (or whatever software I am using to bake my normals into the diffuse) Then I bake the lighting of the scene into the diffuse along with the normals and other properties.
(Beautiful normal map, I know right…)
As you can see in the mesh I made above, the mesh appears as if it has depth; although, in reality, it is flat on each side with the exception of the cracks.
This is how powerful baking normal maps can be to faking depth and making your assets look a lot more pretty.
2. Low poly is a workflow, not an artstyle
Low poly seems to get a bad rap on ROBLOX, but many people seem to understand that ‘low poly’ is not so much an art style, as it is a work flow. If you have great topology, every mesh you make should have the lowest amount of triangles you can possibly make it
Now, the workflow I follow for baking my Normals is: Model low poly mesh → Model High poly mesh → Bake normals in substance painter. The low poly mesh is what you are going to want for your ROBLOX game, however if you are intermediate to advanced as the title suggests, you would already know that.
If you want tips on how to become better with your topology, you can watch these videos. Topology can really help make things like Marking seams, UV unwrapping, and even animating much easier on you; making texturing fun and easy.
I found this video very helpful for beginners^^
3.Get substance painter (or a free alternative)!! 
it seems like ROBLOX mesh makers lack a lot of skill when it comes to actually texturing a low poly mesh. This can be easily overlooked when roblox provides such AMAZING materials!!
No. You can increase the quality of your mesh ten fold by just investing in substance painter. You can even get a free alternative like Quixel. (though I wouldn’t recomend it.)
Substance painter is also amazing for baking normals that can really make your mesh pop in ROBLOX.
4. Use reference images
one program that I use extensively is called “Pureref” and it is amazing for storing reference images that you can look at really quickly. You could also use google images.
What I am trying to say is, reference images and blueprints can really make your model seem more grounded, and you can take a more methodical approach to making it rather than gunning it with a sense of the end product.
I recommend Sketchfab as well, because you can get a 3D turntable of any mesh you could probably think of, which is especially useful for 3D modeling and I use it all the time for reference.
So, in conclusion, ROBLOX might have it’s limitations, but we still should try our best to create well designed and textured 3D assets for our ROBLOX games. Happy meshing!
Let me know if you would like to see anything else added to this list, or any suggestions I could give