1. No more wishes
2. Must be within Riot's ability to grant.
3. No free access to champs.
4. No free skins
5. No RP/IP
Basically, you can ask, for a feature to be implemented in the next patch, a new champ, item, or simply something they could do for you IRL.
Some of the wishes made on this thread have actually been granted within the last several months or so, so give it a shot, you never know!
EDIT: Nerfed rules a bit
EDIT2: Further nerfs added
EDIT3: Added disclaimer
Disclaimer: Riot Games reserves the right to deny wishes at will, so its more like "Riot may grant you ONE wish"