It makes sense to base PM characters off their Melee selves, since this is Project Melee and all, but when it comes to characters who are really really terrible in Melee (like Ness), I think the developers are justified in making more changes, even to the point of total overhaul. You can definitely expect low tiers to play different from their Melee selves, because if they played the same, they would still be awful!
(although it's probably worth noting that I played Ness in B+, not in Melee)
As far as I can tell most of the characters in Melee that we've seen (AKA zelda and Kirby) still do the same things they could in Melee (Except kirbycides of course) and more in PM. I'm not opposed to Ness getting more tricksies that B+ brings, but fair in particular should not be anything like the over centralizing thing it is in +.
I think the ideal picture for Ness should be a Melee base with straight buffs, some smash 64 nods (such as D-tilt and initial dash, I mean, if you can give Kirby a crazy good start dash Ness should deserve one too, if only for testing right now), and whatever B+ tricksies can be added while maintaining what he can do in Melee. Hell, I'll even go as far as suggesting a look at *Shock* balanced brawl's Ness, who does a lot more shield damage with his PSI aerials and Dash attack (PSI Defense down), giving him an occasional poke or rarely a breaker.
Actually, giving Ness some air tight block strings from aerials might not be that bad of an idea. It's not like PK Flash is faster than the average person's after shield break jamming. There's F-smash, but only testing would tell if breaking was too easy and too rewarding.
All in all Ness is a very overlooked character and needs a lot of TLC to patch together properly. I just don't want to see him being B+ 5.0 Ness with Melee physics and minimal Melee aspects otherwise.