@RoboBlue:
Lost World had new level DLC.
Generations was seven years ago, and aside from Mania none of the Sonic games since then have been very successful.
Given that Sega is already trying to milk the success of Mania, I won't be surprised at all if they announce a bunch more DLC at E3.
The Lost World level DLC consisted of a pre-order bonus Nights-themed boss rush which hardly took effort at all besides reskinning a couple assets, a Zelda-themed level, and a Yoshi's Island-themed level.
You do make a good point about DLC levels, but the catch here is that Sonic Lost World is part of a three-game exclusivity deal between SEGA and Nintendo. Sonic Lost World, Mario & Sonic at the Sochi 2014 Olympic Games, and Sonic Boom. Given this occasion, I treat those DLC levels as an exception developed ahead of time to celebrate the partnership deal rather than an indication of SEGA becoming more open to large DLC expansions for Sonic games.
I wouldn't say they're milking Mania so much as getting every bang for their buck from a physical release that they either did not release the first time out of incompetently not expecting such a success or purposefully kept it in the pipeline to milk as much cash as possible because they knew the fan demand for it.