I think Tails could definitely be a good candidate for DLC. If there's multiple characters all at once he may even be amongst the first.
He was a Mii Costume in Smash 4 which has shown to be fruitful for many characters even if they're not playable, (Became Assists or kept their Assist Role if they already were. Tails should at the very least be more then just his background cameo on Green Hill and if that's it for the base game for Tails that might just make it a tad suspicious that there's still plans for him. If DLC continues the Mii costume upgrade route that some of our newcomers already have, it should be a consideration).
Could provide some stock into Smash Ultimate DLC for Sega that we KNOW will be profitable for both Nintendo and Sega. Nintendo boasted how successful Smash 4 DLC was. And Ultimate's no doubt is only going to print more money. Many 3rd parties may want some of their feet in some of that instant moolah, including Sega.
If Shadow misses the Base game, guess who'd be the perfect candidate to be packaged as an echo fighter with Tails for double the Sonic fanservice? (Though even if Shadow is an echo in the base game, I think Tails could still stand on his merits as a good DLC choice anyway)
While many currently think characters from 3rd party franchises already represented by their main character can only be echoes. Reminder that Smash 4 DLC pretty much broke fan written rules like that for each one. If at any point this game breaks a "3rd Party franchises getting more then one character can only be echoes", I'd highly expect Tails as the one to do it. He would definitely fit the bill as a more Isabelle style semi-clone.
And finally, Tails stands on his own as an icon. Being one of the rare characters that could boast a Luigi sort of legacy in they're the iconic Player 2/Sidekick that is nearly as synonymous with their franchise as the main character itself. When you tell me to picture Sonic the Hedgehog in my mind, it's not Sonic alone running through Green Hill Zone by himself. It's him being followed by Tails as the iconic duo they were. He was even with Sonic in that old EGM Melee April Fools prank, which would alone make it a great legacy to include Tails to bring that old prank full circle (Let's just hope Sonic and Tails wouldn't be as hard to unlock as that prank was heh). I'd be willing to bet that Sakurai would even see the appeal of Mario & Luigi Vs. Sonic & Tails being made possible in a regular Smash team mode. Or Ken and Tails, both characters known to be best friends with the besties with current 3rd party characters, Tails amongst the many other younger/kid characters in Smash, Tails and Fox together as two foxes that know how to fly planes really well, etc.
Tails would just fit so much. I don't think he's got a shot at base at the moment. But I definitely think he may have a few things going for him heading into DLC even with other 3rd parties and Switch-era characters to put into consideration. Not going to say he's a shoe-in by any means. But just mentioning him as a decent possibility.
's to have him confronting 
