Sidherat, first of all, thanks for keeping this discussion alive and adding your thoughts and ideas regarding Ball Lightning. It is a skill that is currently undervalued by the community but next league the lid is going to be blown off this skill. There are popular streamers such as RaizQT who plan on focusing on this build, and I wouldn't be surprised if they take a lot of your ideas...
With that said, and after looking at the new 1.2 tree, here are my thoughts for a BL build.
Your 5-L setup is sound, and I wouldn't change anything about it. It is unconventional and exactly why I think people underestimated the gem. This setup I believe can be augmented by curses- TWO curses to be exact. The combination of conductivity, elemental weakness, lightning penetration, and the 2% lightning penetration node is just too good to pass up. A downside that I see is the upcoming nerfs to curses, but with the new curse nodes on the skill tree there might be the potential for curses to be even MORE powerful than they were before. Another downside that I see to this is party play where EVERYONE flings curses around... the solution to this is simply to play solo or with a group of friends.
Arctic Armor is boring. Do we really need to use lightning of purity for max lightning res? Personally, I don't think so. AA is a relic of the past, and while good we can at least explore new things... What about going hybrid? That is my proposal. The new tree has made hybrid life/ES very attractive. The negative behind going hybrid of course is chaos damage bypassing ES. Or, we could go full blown ES/CI but that is boring and I think many people are going CI to begin with...
I know AA is great for witch builds, but I find them absolutely boring. At least with the discipline aura, you can bet that your getting a lot of ES potentially. I can see this character having a life pool of at least 6K.
The witch, I believe, is the best character for this. She is close to the lightning wheel, and there are a lot of nodes that provide damage and survivability. I know that Sidherat starts at the Shadow, but personally I don't see the benefit from the perspective of the new tree. Either way, I'm sure that a viable Shadow BL user will do just fine.
Besides Divinarius and Carcass, I'm not sure what unique items would be best.
If we go hybrid, I'm sure we are going to need a lot of chaos resistance on gear. In addition, it would be nice to have some added lightning damage on gear as well.
Those are just some of my thoughts. I'm looking forward to making a BL character in the new leagues.