Guild Wars 2

Add incentives for remaining with the same server. This will artificially increase loyalty to a particular world, thus reducing attrition and indirectly improving teamwork.

Some examples might be:

• Increase the power of the mists based on how long a player has remained on the same world. For instance, +25% better after one week, 50% better after two weeks, up to a maximum of twice the benefit for more than a month. (The initial bonus might need to be reduced or eliminated.)
• Increase the amount of supply that can be carried, e.g. allow newcomers to carry 5 (10 from influence-enhanced forts), short-termers to carry 10 (15 from i-forts), and veterans to carry 15 (20 from i-forts).
• Increase the participation-based influence gains for veterans, perhaps 3i instead of 2i per activity.
• Offer a Power of Loyalty bonus for remaining on the same world, in addition to the Power of the Mists.

The examples above are meant to be illustrative — the idea is simple: offer a reward for sticking with the same server, even though the balancing details might be complicated.