Some extremely good suggestions there.
1.
Make it buff another heal when you're near it. Say, for example... "When within 15 yards of the lightwell, your subsequent renew casts will heal for 50% more". That's just an example, it could buff any regularly used spell really.
Very nice idea. Also fits well with changes to other healing specs such as Druid's Efflorescence and Paladin's new aoe heal. Which means its balanceable.
2. Make the lightwell HoT a spell cast by us that is only useable when Lightwell is active. This removes the mobility issue, keeps the healing and increases the convenience.
Balanceable though it may mean having to manage a Lightwell like a short cooldown paladin buff if the HoT turns out to be too useful to not use all the time. Also, the more powerful this HoT becomes the less of a reason to play the resto druid ("hot") healing spec, imo. There's already lots of reasons to not/no longer play a resto druid come Cata compared to the other healing specs.
3. Make it a pulsing healing-stream-esque object that heals all targets near it for X amount every X seconds.
This is far too Shaman-esque and also mimics new Paladon aoe heal. I cannot say this is unlikely to happen because as well all know Blizzard likes to cop out and do the easy thing every once in a while (just see all this homogenization already going on with all healing specs and tanks for Cata).
4. Make it duplicate your heals for X seconds. Effectively, the Lightwell becomes a copy of you, though perhaps doing 50% healing to keep it balanced.
Very cool! Sort of the Dancing Rune Weapon of healing. While I think this would be fanastic to have, it would basically require Lightwell to become end-of-tree talent. It would be hard to balance if it has access to the full or almost full reportoire of priest spells. It would also need to be a smart healer (no overhealing kthx!).
5.
Make it so that you can have upto 5 lightwells at once, and they work just as they do now (with adjusted healing) and a fixed amount of charges each. It should no longer drop target. By doing this you make the mobility issue less of a problem because you can spread out your lightwells, and people can use them at their convenience without dropping target. In addition, it adds an interesting "lightwell management" aspect to holy, since they'll be despawning upon used up charges. Perhaps it would trigger a lightwell cooldown after putting up 5.
Very nice idea. I love this since it seems much more fun than 1. It would also introduce a new player skill element by the placement and timing of the wells. I guess the problem is one of balance. This spell alone would be a game changer in certain situations and would be both tricky to balance for Blizzard and hard to predict the optimal way to use it for a new hard encounter for theorycrafting raiders. Of course, its these difficulties that make it so appealing
TL;DR. I think you're right. Blizzard has and has had many options with this spell, especially with a new expansion on the way. It's pretty shocking they have done nothing useful with it in 5 years.