The Resounding Thunder feat is the most widely used solution.
First you have to find a way to add Thunder to your attacks, the easiest is probably the 16th level feature of the paragon path Malec-Keth Janissary.
Another, in my opinion inferior option it the Master of Flame paragon path. Both get bigger bursts at level 16, but adding fire is difficult.
Optimizing Wizard at-will powers if a wasted effort, Wizards get some of the best Encounter attack powers, all of them Standard actions. By the time you could get White Lotus Master Riposte, you have 4 great Standards. If you are indeed optimized, the fight is usually over in 3 rounds. When would you use those at-wills?
Optimizing those encounter powers with Enlarge Spell for example will help At-wills as well, just do not concentrate on at-wills.
You can choose from different builds:
- If you want damage a Genasi with the Elemental Empowerment feat is quite strong from early heroic.
- If you want control, you have to take different Encounter powers and feats, but it is really strong from Paragon.
More about the possible builds you can find in the Wizards forums.