why would he have to be evil?

Well Canonically the other doctors had to fight a villain called the Valeyard - who was evil, or at any rate trying to kill the doctor and large numbers of otehr people - and turned out to be the 13th doctor. And supposedly the new show is continuous with the old show. Of course they can simply decide that it does not apply - but it seems like they try not to contradict previous canon if possible.

*head spins*

So, the 13th regeneration ends up going evil, and going back in time trying to kill his older selves, amongst others?

Would we see the munching gargoyles from Father's Day, with that kind of crossing back on his own timeline?

Or maybe he discovers that he played a key role in the Time War, and the only way to undo the genocide of so many races and planets is to take an earlier version of himself out of the equation. Killing a few, but saving billions? Hmmm...

From what I've been reading (specifically in regards to what Dualla says about him in the season promo), it almost feels like the reasoning behind why he got fat came after the offhand decision to make him fat.

Oh, huh. That would explain why it's so damned sloppy. Whatever, man, I get enough of a headfuck from the rest of the show, I don't need Lee to randomly be overweight too.

And supposedly the new show is continuous with the old show.

I don't know, man, to me it seems like they're not making any particular effort to keep things in sync with old canon. And old canon is large and wily and strange and didn't make sense to begin with, so I'd probably leave it behind too. Besides, they'd come up with something before they hit the 13th regen stage. I can't honestly imagine that DW will be on for that long, really, despite the length of its previous incarnation.

from Wiki

In the Sixth Doctor story arc The Trial of a Time Lord, a Time Lord with the title of the Valeyard (played by Michael Jayston) was revealed to be a potential future Doctor, existing somewhere between his twelfth and final incarnations and embodying all the evil and malevolence of the Doctor's dark side. The Valeyard was defeated in his attempt to actualize himself by stealing the Sixth Doctor's remaining regenerations, however, and so may never actually come to exist.

*wipes forehead*

Whew!

Does anyone else watch the BSG preview "on demand"?

The Master and Lord Berusa also apparently went power-hungry and evil in their 13th incarnations, so there's at least a trend if not a hard-and-fast rule.

Am I do only one likes the idea of a season (or at least a few episodes) of an evil Doctor?

Well, it makes a certain amount of sense. You're an ageless being with the ability to recover from death. Suddenly your safety net is gone and the next time you die, it's for good. That's going to have a major psychological impact. It wouldn't necessarily turn one "evil" but it would probably lead to a major re-examination of how one lives their life.

Am I do only one likes the idea of a season (or at least a few episodes) of an evil Doctor?

Like I speculated before, I could kinda see it happening as a last-ditch effort to enable some greater good to come about, but we've just seen the Doctor come up against the temptation of saving his people and he refused (thanks to SJSs influence), so barring the "I'll kill myself and a couple dozen to save billions", I can't get behind an "evil" turnabout with his character. I don't see it as feasible, and also wouldn't be interested in watching such a black character development. Grey is far more interesting. "Doctor Who goes evil" is just too much mustache-twirling, and I can't see any legitamacy behind it.

Suddenly your safety net is gone and the next time you die, it's for good. That's going to have a major psychological impact. It wouldn't necessarily turn one "evil" but it would probably lead to a major re-examination of how one lives their life.

exactly. That would take us to a fascinating grey/morally-conflicted area, but the logical outcome most definitely would not be black/evil.