Enhancements
Continents form super-continents much more readily and organically, and stay together longer before splitting
Hotspots can now have a range of scales and their activity fluctuates more over time
Removed inertia from plates so their motion is now directly driven by mantle convection dynamics, being much more strongly pulled by colder subduction boundaries and moderately by divergent and collision boundaries
The number of active plates can now fluctuate between the value on the slider and half that value
Plate splitting is now dependent on a range of factors and probabilities (the split axis is still random, as a more realistic modelling of fault-lines would be significant work), favouring predominantly oceanic plates and allowing for large plates to exist for longer
Orogeny over subducting plates reworked so that mountain forming is highest a bit in-land, and not immediately at the plate edge (it takes time for the subducting plate to sink and melt so volcanism is higher further back from the boundary)
Reworked crust density evolution to model thermal cooling, compressive loading, and hydration
Bug Fixes
Fixed plate center-of-mass calculation that resulted in plates tending towards being roughly evenly distributed about the equator
Fixed plate velocity bug which meant almost all plates were moving at their maximum velocity almost all the time - plates now have the intended variety and fluctuations of velocities
Fixed excessive orogeny at collision/subduction boundaries causing extremely tall mountains
Fixed a complex suite of subtle bugs causing generally excessive continental crust formation
Fixed a bug that created crust with oceanic densities but continental volumes, resulting in what appeared to be continental crust being subducted
Fixed a viscosity bug that prevented plates being as rigid as intended (they are still quite deformable at the simulation timescales though). I'm still not 100% happy with plate rigidity in general but further work here will need to be a lot more sophisticated to remain performant