As said here on the official ESO Forums:

Below is a list of all the tests we are currently planning for Cyrodiil on PC, along with the dates and a short summary of what to expect. Note that each test is independent of each other, and tests are subject to change. We’ll post in this thread once each test has started or ended, or if anything changes, and will keep it locked from comments so it’s easier to follow the current status. Remember that you’ll be able to earn double AP in Cyrodiil during all the tests!

Once all these tests are complete, we’ll spend some time analyzing the data and determine next steps, which we’ll be sure to share with you.

  • September 8-14: Shared global AoE cooldown, 3 second timerFor this test, all AoE abilities will have a shared global 3 second cooldown. This means when you cast an AoE, you will not be able to cast another AoE for 3 seconds. For example, if you cast Ritual of Retribution, you won’t be able to cast Puncturing Sweep for 3 seconds. For this test, all AoE abilities will have a shared global 3 second cooldown. This means when you cast an AoE, you will not be able to cast another AoE for 3 seconds. For example, if you cast Ritual of Retribution, you won’t be able to cast Puncturing Sweep for 3 seconds.
  • September 14-21: Individual AoE cooldown, 3 second timerFor this test, each AoE ability will have an individual cooldown. This means when you cast an AoE, you will not be able to cast that same AoE ability for 3 seconds. For example, if you cast Puncturing Sweep, you won’t be able to cast Puncturing Sweep again for 3 seconds. For this test, each AoE ability will have an individual cooldown. This means when you cast an AoE, you will not be able to cast that same AoE ability for 3 seconds. For example, if you cast Puncturing Sweep, you won’t be able to cast Puncturing Sweep again for 3 seconds.
  • September 21-28: No cooldown, global ramping AoE costFor this test, each AoE cast will have a global ramping cost. Each stack of the debuff increases the cost of any AoE cast, similar to how streak or dodge roll works when casting an AoE, and you receive a debuff for 5 seconds. For this test, each AoE cast will have a global ramping cost. Each stack of the debuff increases the cost of any AoE cast, similar to how streak or dodge roll works when casting an AoE, and you receive a debuff for 5 seconds.
  • September 28 – October 5: Individual AoE cooldown, 3 second timer and global ramping AoE costEach AoE ability will have an individual cooldown from Test 2, but also combines that with a global ramping cost from Test 3. Each AoE ability will have an individual cooldown from Test 2, but also combines that with a global ramping cost from Test 3.
  • October 5-12: Group size is limited to 12 in Cyrodiil, and any ally-targeted abilities are only applied to group members
  • October 12-19: Group size is limited to 12 in Cyrodiil, and if a group contains 6 or more players, everyone’s AoE abilities will go on individual cooldowns and have escalating cost similar to the tests above

Thanks in advance for participating in these tests. The more players that jump in, the more data we’ll be able to collect and analyze, which will ultimately give us better results!