I don't think I had written this down yet, so I'm going to go ahead and summarize my thoughts here. I want the Wither to be an interesting post-Dragon boss, in the form of a boss rush.
Per #142, wither spawn eggs will be disabled and will not spawn as a result of events.
Once you defeat the ender dragon, I'll increase the drop rate of wither skulls significantly, so that you can summon the Wither more easily.
The Wither himself summons other mobs as part of his attack pattern.
At high HP, he summons weak common mobs like silverfish and bees.
At medium-high HP, he adds in sturdier mobs like zombies.
At medium-low HP, he adds in mobs that themselves spawn other mobs, like skeletons and blazes.
When he gets low, he straight-up starts summoning boss mobs like the warden and ender guardians.
And, of course, the Wither will passively heal during the whole fight.
I don't think I had written this down yet, so I'm going to go ahead and summarize my thoughts here. I want the Wither to be an interesting post-Dragon boss, in the form of a boss rush.