Some years ago, I got to playtest GURPS: Vampire.
GURPS isn't my favorite system, but it was one of the various games our group played. Besides, I had never playtested before, so while I grumblingly went along, I had a blast doing it. I made a character that was rules-legal, but conceptually odd. He just couldn't be easily pigeonholed into either of the families he most closely resembled (Brujah and Malkavian). But the game didn't break. My design only created role-playing problems, not systemic problems.
Based on my experience, my advice is this: PUSH THE SYSTEM. This is where both your powergamers AND your master thespians will shine like no other time. The powergamers will be able to find the breaking point in powers and abilities; what things in the game will unbalance a campaign and how. The master thespians will be able to see where the rules don't support certain character types, something that matters especially if the PC type actually is game appropriate. When you push the system to extremes, it becomes easier to find the flaws that exist in the system.
For example, the first edition of Palladium's Heroes Unlimited focused on medium to low-powered superheroes. However, a flood of players kept demanding the ability to play Superman wannabes, so they had to add the Mega-hero class to the game when it got revised. If that critique was something that showed up in the original playtest, Palladium did itself a disservice by ignoring it. If it didn't- the playtesters who shared that opinion didn't do Palladium any favors by not mentioning it.