While at one stage it was announced that you would be able to invite people to visit content they haven't bought, this is no longer the case.
On the other hand, expansion packs will be entirely optional. If you buy the first and third expansions, and a friend of yours buys the second and third expansion, you'll be able to play the expansions yoy have in common together (i.e. the third one).
Also, ArenaNet plans to add a larger variety of skills in the expansions, but not more powerful skills and items. So you will still be able to compete in PvP.
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While at one stage it was announced that you would be able to invite people to visit content they haven't bought, this is no longer the case.
On the other hand, expansion packs will be entirely optional. If you buy the first and third expansions, and a friend of yours buys the second and third expansion, you'll be able to play the expansions yoy have in common together (i.e. the third one).
Also, ArenaNet plans to add a larger variety of skills in the expansions, but not more powerful skills and items. So you will still be able to compete in PvP.