As of the latest patch, we now have changeable difficulties for The Story and Tarot missions. Done by PL, he thinks it a good idea, but if the Newgame+ is planned (if) and it should be, then...
Bad approach is having a fixed multiplier for damage/hp. No matter how good the initial numbers are, the same can't cover all creatures (well, if difference is small, say <50% then it doesn't matter). Results are hordes of glass cannon 1-hit-kill (or die if not) and thrillingly boring 20min boss fights, where nothing happens. So, extremes are problematic - bosses have huge HP/dps ratio and glass canons the opposite.
By being a balancer in number of mods, Snakefist approves usage of geometric mean for both parameters. It is a mathematically proven and practically proven method, and would look like:
1) glass cannon: standard for intended 200% HP / 130% dps = gmean(2.00*1.30)
2) normal: standard for intended 200% HP / 130% dps
3) boss standard for intended 200% HP / 130% dps = gmean(2.00*1.30)
So, three curves to rule them all. As driving mechanism behind PL is probably a multiplier, think of using geometric mean, which makes most units mostly balance by pure calculation, and manual tweaking where needed.
Newgame+ and why to have it - yes, it's uncertain because we don't know the story, but!
Characters will finish The Story probably by some low level. D2 had normal difficulty finished with 25-30% total exp. Next difficulty level wasn't the same but just more HP/dps, but also adapting to different conditions - suddenly protection become alot more important, mobs had elemental (or physical) resistances, immunities even, so not all builds were viable - but another level of weapons (the same types, increased stats), plenty of levels to fix the build for more persistent players. Far more different than, for example, Torment 24 in D3.
Another reason is superior story-telling NeoCore usually has. By having small changes, like alternative choices or a event that didn't happen but did in original story, we can have different gameplay and different Story feeling - something more.