Dimensions

Scale your mesh appropriately according to the metric unit system in Blender. For reference, Fluf is 1.8 meters.
FV_IPAllSizes

Deltas & Transforms

When either creating assets in Blender or importing the assets from different 3D applications, make sure all delta transforms are fully applied (position, rotations, scale) and have the respective values pre-applied [0,0,1] this helps ensure the best output format and undesired glitches, especially on deformation meshes.
DeltasTranform

CTRL + A = Apply all Transforms

Edgeflow

Ensure your edge flow caters to the desired animations
Keep in mind that every project will have unique edge-flow specifics to their requirements, but when it comes to humanoids keep it generic.
edge-flow
For most projects, we use CC4+ (Character Creator 4) as a base and manually modify the topology so it caters to our needs.

Symmetry

Keep your mesh symmetrical for as long as possible.
symmetry

Tris/Quads

While modeling utilize a triangulation modifier to preview your mesh in its exported state. Quads and Tris are fine, just be conscientious of their use regarding edge flow. Apply triangulation before export to ensure consistent shading between different engines and software.
Triangulation

Seam Considerations

Backfaces

Delete every face that’s not visible inside the geometry. A good example would be the thickness of the jacket or T-shirt. Instead of adding thickness, you can collapse edges to reduce polycount.
backfaces-1
backfaces-2
backfaces3

Normals

Orientation of face normals should in most cases point outwards. Exceptions can be made if you decide to use single-sided meshes with Backface Culling deactivated. Keep in mind that opacity shaders will require support.
BackFaceNormal