What worse for franchise: tiny upgrade, deviation, dumb down, "inbreeding"?

[Jaos]

What's wrong with tiny upgrades, if they're there? Provided the game is any good to begin with, that's the way to go.

Also, as long as the original game series isn't abandoned, an additional well thought out spin off is ok. If you'd always reject that, you couldn't play Mario Kart or Paper Mario.