Disagree that Karma is a bad support, in my experience most people that are used to enchanters like Janna, Lulu, and Soraka don't know how to leverage her unique ability to aggressively win lanes. That said, her winrate and the way pros keep leveraging her primarily as a midlaner DO point to a certain lack, I'm just afraid that she's going to get "The Lulu Treatment" and attempts to buff her supporting would cause her to become meta pick/ban in mid, not a desirable result given most of what you list here would be straight buffs.
The problem with Spirit Bomb is that it greatly multiplies the number of affected targets. I miss it too, but they've buffed her Mantra CD reduction by A LOT since that was a mechanic, so bringing it back would likely mean that would have to be nerfed, resulting in less uptime when not in a hectic skirmish. Likely result is that it's completely wrong to mantra anything but E in a teamfight, something that is already situationally true most of the time; the primary tension of Mantra involves being tempted to use it for damage, and if you actually tagged multiple targets with both explosions the mantra CD could be even lower than after a 4-ally shield.
Healing is very hard to budget unfortunately, power-wise. How much shield strength would you be willing to cut in exchange for that sustain? Shields in general are too strong right now and are slated to be nerfed in duration or changed to quickly decay. See Irelia's new passive and the talk of an item that shreds shields! Have you tried opting into this playstyle by building Athene's? It's small I know, but Karma appreciates that little heal the same way Soraka appreciates the little shield she can spam from Aery.
I'd like to see an ally mechanic for her tether return personally. The 'clothesline' effect that was on it back in the day is too hard to use, but something totally different like amping ally damage or transferring some of her mana could be power that wouldn't break her solo lane.
Folding her current passive into R to fit a new mechanic in is a good idea though! The power budget for that would be much freer if you made it a passive that benefits support Karma without adding power to her Solo lane- Think like how you need an ally for Leona's passive to function, something like that! Got any ideas?
