We decided in our group that the players would run an all-Kineticist party, and that I would run a series of challenges tailored for them. One scenario would be an average dungeoncrawl with a lot of easy encounters, a scenario which involves roleplaying and investigation, and a straight-up severe encounter for stress-testing. I'm building the PCs with input from the players (one wants to be a healer, another wants to have a lot of utility tricks, etc.), as well as the encounters (no demons, angels, or dragons).
What are your plans for playtesting? Are you going to run a module or adventure for the players? Are you going to have a mixed party of other classes; and if so, will you look for synergetic classes? Are you going to include things like Victory Points and exploration roles?