Ever since Shadow's game and 06, Sega's been afraid to take the series in a more serious direction.
They don't understand that it's not a darker storyline we hate.
It's a ****ed up darker storyline written horribly AND ****ty gameplay that we hate.
Sonic 06 would have been good sans hedgehog-on-human-action and a lot more time in the oven. We'd probably think it was the best Sonic game ever made if the devs gave a **** and didn't rush it.
Forces was the first big attempt in a very long time to make a Sonic story a bit darker, and though the gameplay wasn't the greatest, it still sold pretty well. And though the slightly darker atmosphere should have been handled better, I'm sure it was seen as a breath of fresh air by a lot of people. I was so sick and tired of Sonic games being written with asinine dialogue, only focusing on Sonic as a playable character and way too much even story-wise.
Don't get me going on how much I hate Lost World's writing and story. The game itself played decently but holy hell was the writing, characterization...EVERYTHING pertaining to the plot just straight up garbage.
Shadow being playable for the first time in ages was received very well as far as I could tell. Sega did so much to play up and give fan service to Shadow fans - I NEVER thought they would do anything to reference Shadow's game again, but here we are with hip new remixes of the Shadow menu theme and Westopolis!!! Like holy **** I don't think some people realize how HUGE that is. I'd love a Shadow sequel but it won't happen.
Shadow was also written and portrayed pretty well in Forces. He's badass and stomped Infinite in epic fashion without being over the top, emo edgelord like he was in his game. He actually had a reason to be there. It's like Sega paid attention to what fans wanted for pretty much everything in forces, down to the OCs...EXCEPT core gameplay.
With all that being said, if Shadow's not in base, I strongly believe he'll be one of the marquee DLCs, given how popular he is, the raw fan demand for him as an echo, his Nintendo and Sonic history, and Sakurai's familiarity with the character. SAKURAI PLAYED SHADOW TO GET INSPIRATION FOR SONIC'S MOVESET IN BRAWL. He is VERY well aware of Shadow - he put him in as an assist for two games straight and now Shadow is nowhere to be seen for a VERY long time. There's NO way that's not intentional on Sakurai's part. Unless Sakurai just wants to do what he does best by disappointing me, he's kept Shadow hidden for a long time and will reveal him as base or DLC pretty soon.