Arenanet is introducing Fractal Random Mistlock Instabilities in the upcoming patch.
A message from Benjamin Arnold, Content Programmer – Fractals / Raids:
Mistlock Instabilities were designed as a way to add variation to gameplay and difficulty when players are repeating the same fractals multiple times. The intent was to provide unique challenges and promote new playstyles based on which instabilities are active for a given fractal scale. Unfortunately, the original implementation did not quite succeed at this.
Initially, instabilities were hard coded for each scale, and would change only if we manually altered them on patch days. As a result, rather than feeling like the gameplay varied at Tier 4, you got the same set of instabilities every time you did a fractal, and it got stale, or worse, felt consistently un-fun. In addition, the manual nature of the instability list was hard for designers to maintain.
In the very near future, instabilities will be randomly assigned to each scale, and will change each week, with one instability at Tier 2, two instabilities at Tier 3, and three instabilities at Tier 4. Instabilities will be enabled via a per-map whitelist, so designers can easily update which instabilities will ever be available for a certain fractal.
We believe that this added variation will improve the fractal experience for dedicated players, and help keep daily achievements from feeling stale or unchallenging. Going forward we will continue to look for new instabilities to add and improvements to implement, in order to make the system more enjoyable and understandable.