Eventually is so far down the pipe-line it feels like it doesn't matter, will I still care about this game in 2-3 years. At the pace we are going, we get 4 VGUs and 2 class updates a year.
Assuming all goes as planned. Galio is next, then Urgot and then Eve. All of which should be done by next fall I assume. There are still big contenders for updates by that point like Akali, Irelia, Morde, Swain, Kayle/Morg, and a bunch others. She could slip through though, her groundwork is done for them. Poppy and Trist set huge visual goals for Lulu and the concept shouldn't take very long. We know what a yordle looks like, and Lulu already has big features that divide her.
Splash arts like Pool Party Lulu and some fan art do a good job of portraying her if she didn't look like a mess:
https://3.bp.blogspot.com/-1JUaI2oDC5U/WJo9TsE4aXI/AAAAAAAAdeI/Yk6q6Py6IzAzK01EaPE7mpthpzdsZb1bwCLcB/s1600/Zac_Splash_2.jpg
http://orig06.deviantart.net/005d/f/2013/291/3/0/lulu_league_of_legends__knkl_by_knockwurst-d6qwx6p.jpg
Currently in game though she has a VERY strange idle pose that is made really obvious by her SG skin:
http://puu.sh/tTn06/35b9d27f52.jpg
It's so annoying cause the character seems so vibrant and full of life and their in game is just really dull, it just doesn't do her justice.