So, checking out the 1.13 snapshots, I realized that with Channeling, you can easily strike almost any mob in the overworld with lightning. Looking at the list of mobs transformed by lightning, it's a decent trio of mobs.
Pig > Zombie Pigman
Villager > Witch (which is an advancement now!)
Creeper > Charged Creeper
But then I absentmindedly threw a Channeling trident at a Zombie Villager, and I began to wonder...
What other mobs could transform when struck? Now more than ever seems like the time to ask, with this on-demand striking power at the cusp of minecraft's survival mode...
Here are a couple of quick, interesting ideas.
Since I mentioned it, I may as well start with it. Zombie Witch (or Necromancer)
Made by striking a Zombie Villager with lightning, this undead practitioner of dark arts retains far more knowledge of potions and magic because of all that empty space in a zombie's brain, leaving them with a mind stuffed full of mystical know-how. They'll throw far more complex potions, sometimes even Lingering Potions, and won't hesitate to heal up any undead around them with Harming Potions, targeting low-health undead. This even includes the Phantom, which is just about the absolute worst thing ever. The Necromancer will even throw Strength splash potions at its undead allies. Some have even said that, during the lightning storm, Skeletons surrounding a Necromancer are turned into their far more hellish relatives, Wither Skeletons, which charge into the fray with their stony blades thirsting for blood.
Uses: Drops advanced potions when killed, and has a chance to turn normal skeletons into wither skeletons, which can drop Wither Skeleton Skulls.
Lesser Wither Skeleton
And of course, there's the idea of Pigs turning into Zombie Pigmen, why not turn the overworld's skeletons into Wither Skeletons? Of course, I'm not just suggesting this so that skeletons drop coal, no no. I'm suggesting this because it would be an easy way to get Wither Bones. Yes, I know, there have been suggestions and mods to replace the coal/bone drop with a Wither Bone drop, and this is exactly what this entails. Give Wither Skeletons and Lesser Wither Skeletons a drop of Wither Bones, and remove the bone/coal drops, while Lessers can't drop skulls. Burnt skeletons shouldn't drop unburned bones. But I digress. Lesser Wither Skeletons would retain their bow, but with a high chance of it either becoming enchanted with the enchantment of Flame. Note that a Necromancer DOES create full Wither Skeletons, NOT Lesser Wither Skeletons.
Uses: Drops Wither Bones, whatever those end up doing.
Those are my current two ideas for additional lightning-struck mobs!
I like the idea of turning zombie villagers into different mobs, but the necromancer kind of feels like a better version of the witch. In my opinion, witches are too easy to deal with as is, so maybe they could be changed to have the behaviors you described. Instead of necromancers, maybe zombie villagers could turn into different types of illagers! (While keeping the same drops as zombie villagers, so this doesn't become OP)
This would be a new mini-boss. It would be a giant spider, being 2 blocks tall and 4 long, and has a orange color in its eyes. It would attack like a normal spider, but would shoot a web also, happening every 8 seconds. It would drop the normal drops, with a 30% increase in drops, as well as webs.
Good idea or bad??
I have posted a thread about this incase you want to learn more