Update WPF to use Direct2D for rendering instead of Direct3D 9
WPF currently uses Direct3D 9 for rendering. This is because WPF was designed when it had to run on XP. Today, all platforms that WPF runs on have Direct2D support. Direct2D has vastly superior rendering performance compared to WPF's current Direct3d-based rendering.
It would thus make excellent sense to migrate to using Direct2D for rendering. As rendering is pretty abstract in WPF, this could probably be done with little to no impact on compatibility, but with HUGE performance benefits. Watch some of the comparisons between Direct2D and WPF.