My idea would be to grant the Eladrin a Wizard At-Will as an Encounter power, but this will be less useful for wizards (maybe instead grant him an extra at-will - as at-will), and it quickly becomes obsolete for a non-Wizard.
That's why I say chose the wizard at will and grant wand proficiency to use the power with. They already get longswords.
Go the whole "All eladrin learn a little magic" route. It's balanced with humans and half elves because it forces which class and power you get.
Human- Current class' at-will.
Half elf- Any other class at-will as encounter
Eladrin- Specific at-will of specific class
Non-wizards get another ranged attack, Wizards get a bonus at will. Everybody wins!