Ok so big rant incoming:
While i do agree we are not gonna get 2 Sonic newcomers,if there has to be one of Sonic's supporting casts imo IT HAS TO BE EGGMAN, (if he isnt in i will still respect Sakurai's decision) Here are a couple of reasons
1: He is one of Gaming's most iconic and biggest villains, he is one the same level as Bowser and far exceeds Ganon,Dedede and K.Rool in terms of recognition. I do think for Sonic characters Sakurai would at least CONSIDER HIM.
2: he is THE SONIC VILLAIN, while i understand that Shadow can be made much quicker and easier, i do think that the good doctor can fit in incredibly with the Heroes-Villains theme, think about it we got:
Mario-Bowser
Link-Ganon
DK-K.Rool
Kirby-Dedede,MK
and you are telling me Sonic-Shadow would be the best fit for one of Gaming's most recognizable and important characters AND NOT BRING IN THE GOOD DOCTOR, thats just disrespectful towards sonic's legacy
3: wouldnt it be cool to do a Mario-Sonic-Dk-Link V Bowser-Eggman-K.Rool-Ganon 8 Player Smash
4: He has TONS of moveset potential with all the mechs,equipment and Armies he has had over the Sonic Series, a FARCRY over the Echo and 'isabelling' ( giving tails the issabele treatment) that Shadow and/or Tails would have some people say some of his mechs are too big AND I SAY RIDLEY MADE IT IN SO SIZE NO LONGER MATTERS.
5: HE APPEARED IN THE 1ST GAME. it should be imo THE ORIGINAL 2 FROM SONIC 1.
6: We could even get an DR.Wily Echo fighter of him (or reverse idc too similair).
7: Sonic is really lacking Representation,(ik 3RD PARTY but no Mania stage come on) all we have is sonic as a character, Knuckles as an assist, and 2 stages thats IT, not even items.
Overall i really think Eggman has a lot more going for him to make it in then tails but Shadow is more likely.
9: I AM THE E.G.G.M.A.N... that justifies him completely