About Witch-King's damage and powers with or without the Ring, if you use him wisely,target him head on in massive battles and single targets ( mostly heroes, monsters and siege ) he is quite good, also he have a wide section of hero buff which is quite useful and his last ability may save an entire battle or even himself if he is low on health
And about the proposal from The Necromancer about his Ring ability to mount a Fellbeast:
I personaly agree and like it because you will have to change only that option ( let sey once he has the Ring he couldn't mount he horse anymore instead he recieves flying mount ) and by the way those who still like the dragon summon will have now a Hero that actually has a flying steed and those who prefer troll summon will have access to them even when he has the Ring.
About the Fellbeast you could also use normal one insted of the armored Mordor version for Witch-King.
But if he is able to mount, he should also lose most of his abilities which is quite a tricky choice: will he harass them from above or stay on the ground and cast spells on the enemy troops ?
Those are my pros and cons. I think that this is good idea ( personal opinion ) and that everyone will state their thoughts and opinion about this
Have a nice week,
Filip