It shouldn't be introduced at all. Nerf the champs, that are op, because of their shields. Janna is hardly the strongest support at the moment. Yea i know she got good winrate, but only because her popularity is super low, and only those who are really good with her play with her. I haven't seen a good Janna in gold2, and below so far, and when i play with her it feels super weak compared to Lulu, or Soraka.
Lulu is strong, but it's not her shield, that makes her strong. It's the passive when put on champion that relies on attack speed. Combined with ARDENT+W it gives serious damage boost to anyone. Lvl18 Lulu gives 118 magic damage on hit with 0 AP. 118 extra AD for basic attacks is very significant even for a Master Yi, or a Varrus. Or she could give up W to nullify an important enemy. It does wonders to champs like Kata, or Zed. However Lulu has one main weakness. Her buffs are very dependent on any ally, that uses attacks speed as main damage. If there is no such ally, or that ally is noob, then her efficiency is greatly reduced.
Karma's shield is the lowest among these, and she also not the strongest, because of it. It's the movement speed that comes with it, and her strong damage on Q. If you were alter her shield to defend a single attack regardless of strength, and keep the speed buff, then she would be almost same as now. If Rene can get into melee range to her, then she is already lost. Her teammates are a different matter, but non-tank teammates will not be saved by her shield, if Rene is in melee range.
Lux shield is an interesting one. It has considerable value, but it takes some skill to use, and time to travel. She won't exactly counter Rene with her shield. More like with ther Q. If that hits, then Rene gona stand around for a long time.