I mentioned this in another thread, but it's possible to make shield healers about shielding missing health, rather than shielding above the cap as their basis. SE seems to be adamant about preventing us from being able to get high levels of eHP, but unless if your shields can be pretty strong, it'll generally feel bad to play a true shield healer.
This combined with maybe migrating HOTs to shield healers and pure healers just have high power and efficient normal heals with some other sort of limitation would make true shield healers both more interesting to play, and not too difficult for new players to pick up.
There's barely anything left to be gutted at this point. Losing the DoT we've got will barely make a difference, though losing our Broil and equivalents will cause an immediate mass migration unless if SE includes a healing or support rotation just as complex and fun as some of the best DPS...that can be done at all times of a fight without feeling like we're just pressing buttons for the sake of being busy.
Personally, such a thing is close to impossible, and SE has repeatedly proven that they are terrible at making interesting and fun but balanced healers.