I think the pet that casts slow idea is an amazing idea especially with all classes getting access to pet hold (no unwanted slowing in exp settings where you want the DS to do its job)
Combining stat buffs is another great idea (on this note fold shroud of the seasons into Relic: WoN for druids!!!!!!!)
There are a lot of alternative avenues that have not really been explored for healing classes and shaman might be ripe for a small addition. Supplementary things like temporarily boosting mitigation, the middle ground between "regen" and "HoTs", spell runes, short duration buff style expansion of the HP pool (essentially a rune that can be refilled by healing), lots of stuff that isn't just upping some heal numbers.