What should expansions do to the level cap?

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A successful MMO gets expansions -- it's one of the oldest truisms in the genre, and even as free-to-play games begin to ease in on the market, it remains the prevailing expectation. Of course, the expansion also comes on the heels of several players having already reached the level cap, which presents designers with an interesting conundrum -- how do you get people interested in the new content you're selling?

World of Warcraft's choice has routinely been to increase the level cap, making a new rush of content for players to level through before populating the endgame with roughly the same diversity. Other games -- including Guild Wars, Final Fantasy XI, and City of Heroes -- broaden the level cap, adding new content that players can work through without necessarily requiring a new bout of leveling.

So which do you prefer from a retail expansion? Do you like the model of adding another group of levels to move through, obsoleting the old level cap but letting everyone catch up to the same point? Or do you prefer an ever-larger endgame with the same cap but more options when you reach it?

TH

I hope the next DCUO expansion will crank up the cap to at least 50 as I always like to have more points to "spend" on power/skills, don't matter to me if I might not ever use them, but hey its a game afterall, and I hate dillemas, so I like to indulge myself.