Brand new code, rather than adapting existing code... which doesn't always work as easily as you'd like...

Adding the features you like.
Removing the features you dislike.

Trying something new, that is an innovation which none of your competitors offer...

There is definitely something to be said for a new version of the same game.
Or for a totally different game, but still the same genre (MMORPG), by the same company.

Blizzard would have to seriously alienate their current player base in general (it was several years of progressively worsening customer support in EQ1 that caused me to switch to Wow).
Or decide boxing was no longer allowed.
To get me to look at another game; sure I take breaks, as long as half a year at times, but I've always returned to wow.