Ness was only considered to get cut for Lucas in Melee, but there's nothing to suggest the same for Brawl.
I think Ness having a significant role (or at least the same as Luigi) in SSE shows that he was planned to be in for sure, with Lucas. Considering that the last minute addition Sonic and the three low priority characters (Jigglypuff, TLink and Wolf) weren't in the main story in SSE, I think it's at least something to go with. Ness is also the first character you unlock, so there's that.
I can't wrap my head as to why they'd cut Lucas to be honest. He's left his mark in Smash, and he's different enough from Ness, so I don't see any reason for that, and yet the vague Gematsu leak is pretty worrying. *shrugs* Hopefully Mother stays with two characters.