uhm... activate downvote shield
First and foremost, the next class update has been hinted to be divers but after that one an enchanter update could be beneficial, I agree, though, I don't see those 4 as overlapping champions, like, at all :S nor are they generalist in any kind of way let me explain my reasons:
-Being enchanters all of them have a heal/shield, though each one of them have very specific use cases or "niches" if you will
-Karma happens to be tankiest on that bunch, she might seem like a generalist because she has high poke and cc but I think it's been shown enough that best Karma is tanky Karma, as a matter of fact, the cooldown on her shield is far more important than her synergy with spirit visage and says a lot more about who Karma really is supposed to be. Followed by Taric (who is a lot tankier though), Karma is the tanky enchanter
-Sona has the highest damage out of all the enchanters, give Sona a regular enchanter build and she'll still do the most damage out of all the class, she also has heal and cc and that's probably why she feels like a generalist but you pick Sona when you lack damage on your team and when you have the chance to scale by poking like a truck. Sona is the mage enchanter
-Nami is all about slows, she's the Ashe of the enchanters, you pick Nami when you need strong heals and plenty CCs, also when the enemy is vulnerable to slows Nami becomes the perfect fit, you wouldn't pick any other enchanter under these situations. Nami doesn't show much on pro play even though she's probably the most balanced enchanter in existence, this is because her kit is pretty unreliable and pro play is all about low risk. Nami is the pure enchanter and an exemplar of how the class should be (more on this ahead)
-Lulu is an odd case, normally you'd say that Lulu IS the generalist enchanter but in reality she's just the duelist enchanter which is an odd combination, the duality of her spells allows her to be seriously dangerous for extended fights, her best case though is as a counter initiator. I like to call Lulu the buff/debuff enchanter
So as you can see, all of them might share some similarities because they are after all enchanters but they play a lot different from each others, at least in the hands of someone who understands their strengths and weaknesses... there is, however, one enchanter that is a generalist and that has been pushed too far into the role:

-Soraka could be designated as the healer enchanter but... all of the enchanters have heals that rivals hers... and they also have methods of survival that far outweigh Soraka's, yes if you Q someone as they dive, you might survive with a sliver of health but compared to CC or even bursting damage, Soraka's performance is simply lacking. It doesn't help that all her scaling is on her heal and that grievous wounds makes you half effective while you kill yourself all the same :S Soraka lacks damage in exchange for stronger heals but... the other enchanters have damage AND heals/shields, as it currently stands, there's no point in picking Soraka but as a pubstomper
I think there is something awesome about the enchanter class, playing it correctly is incredibly rewarding because they kind of have a bit of everything but they only shine when played correctly, Nami for example has very little damage on her kit but if ignored she packs a punch, I can't count how many times I've killed enemies who think is a good idea to dive a lonely Nami, that said, Soraka is the other side of the spectrum, she's been pushed too far as an enchanter and as a result all she can do is heal, even if you build heavy AP all you can do is heal.
If there is to be an enchanter class rework, Soraka needs to be first in line and IMO, the only one :S