It is actually really hard to buff support Lulu without making solo lane Lulu oppressive. For example:
Buff mana/mana regen: She starts to waveclear with Glitterlance. Her waveclear and poke become oppressive without mana gating.
Buff CC: Lulu already has an AOE knock-up with slowing field, an 80% AOE slow and a polymorph. Any buffs give her Nautilus level CC.
Buff cooldowns: She is primarily gated by mana so solo lane Lulu benefits way more.
Buff survivability: Top laners already complain that Lulu is unkillable.
Buff movement speed: Maybe, but her low movement speed (325) makes her distinct from other supports.
Buff AP ratio's: Solo lane Lulu builds AP way more efficiently (magic pen, offensive masteries).
I see most in a rework of offensive E. Make it something interesting that is better with a lane partner. For example increases allied damage against the target or takes away vision for a short duration. Just my two cents ;)
That why mana adjustments on the abilities have to be made in combination with it. Mana sustain is something really important in the botlane. A support that is able to quickly restore mana has the ability to bully a lane. Lulu suppose to be a lane bully back in the previous season. She really need it to bully again. Still a note that currently I made her scaling unchanged what means even with this changes she still only have 16 mana regeneration. That's a pretty low number if you known most other supports have arround the 17/18 mana regeneration on level 18. Actualy she will still have the 3th lowest mana regeneration from all supports!! The only one she jumped over is 
Alistar. Her flat mana is actualy relative ballanced compaired to other supports. Like mentioned this would be a lane sustain buff for toplane Lulu. But I don't think this will be the gamebreaking change that would move Lulu back in the toplane. Not if you combine the Whimsy change with it what would be a hugh nerf for toplane lane. Maybe that combination is just what Lulu needs.
CC is indeed not the solution. Lulu has tons of CC.
With cooldowns I disagrea. Low cooldowns indeed gives a advantage in duel potentional. But this also counts for peeling and that was just what Lulu needed in her lane fase. You have to combine adjustments to create something new so the ability will be used effectly without having to rework her entire kit.
Lulu is indeed hard to kill. For that scenario the Whimsy would open more counterplay. A 15 increased mana cost is a big deal in the early game. Actualy an really big deal. Lulu will have to decide if she will use her mana offensive or defensively. If she tries someting aggressive with her W and it didn't work out she will pay the price. You already saw in the lcs how mana hungry Lulu can be. In the end she runs out of mana and the jungler can gank her for a free kill. Lulu is a verry inmobile champion. Without her W she has nowhere to go.
They already cut her AP ratio's in haf. This is the only reason why Lulu is a simple tier 1 top/tier 2 mid in solo Q. It's actualy only in team Q she's placed higher in the ladder.
The Enemy Cast E is indeed something really powerfull for the toplane and makes bush cheese really hard to pull off. However the revealing part is probaly something that defines Lulu and I'm not sure that it should be removed. I don't think that giving it a leona passive would solve anything on her E. Also any other alternatives would just make her kit stronger or would compleetly remove the function of using the E on enemies.
Taric...