This is a question I've been wondering for sometime....I will start a poll to test the results. However, one could argue that if they made a fully customizable generic game, one would be able to customize their own legends if they wanted too (signiture styles could still be included by 2K and could be chosen when customizing a player). After all if you can re-create Brett Farve, you can likely create Dan Marino as well.
Also, IMO, the legends theme seems like it will be hard to re-sell the next year. While the NFL puts out a game every year with mostly the same players, those players are progressing/digressing, changing teams, coming in and out of stardom, where as a legend stays stagnent. Unless there is a change in the way the game plays, Barry Sanders next year will be Barry Sanders this year. In the NFL game Marquez Colston this year will be a different animal than that of last year's. Plus new uni's, updated rosters, new stadiums, a few new gimmicks, etc.