1) One unit length. The most important update for Blender Next. I found and fixed 13 years old bug in Blender. Now all bones in Blender Next work like joints and have 1 unit length and radius after you Apply Pose as Rest Pose to 3D animation or model. This helps a lot to tweak 3D animations without retargetting them.

Here two demo rig files with Spline IK animation and standart SKM_Quinn model. On second animated rig control Spline is binded to pelvis bone via Child Of object constraint in Set Inverse mode. It's best stable constraint in Blender Next for everyday usage in custom rigs. It also works great in Bone Constraint mode.

2) Better skining system. Because all bones in Blender Next have fixed length and radius.

3) Display Bone Axis scale can be tweaked via Scale slider in Viewport Display tab.

How to start working in new Build? Import FBX file with Quinn or Manny from Unreal Engine 5 go to Pose -> Apply -> Apply Pose as Rest Pose. Tweak joints in Pose Mode and add extra joints via Extrude Disconnected command in Edit Mode.

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Important moment: when you use Spline IK your bones should be oriented to +X or -X axis. You should Apply Pose as Rest Pose twice. Because Spline IK have it's own transformation matrices. Also, you should create all Spline IK rig chains first and skin it later, because your bone positions will be recalculated when you Apply Pose as Rest Pose twice.