You can't blame Sakurai for Lucas looking like Ness I mean that's his canon appearance and Itoi designed him to look that way probably to keep in line with the appearance of all protagonist in the Mother series. Now what would've been nice is if they added alternate costumes like Lucas's pajamas or Pigmask outfit but it seems for some odd reason Sakurai isn't big on costumes. Also in terms of special moves, both Lucas and Ness don't have any special thing about their PSI abilities at all really. They've always been support-like characters with one really powerful offensive PSI, and that's especially noticeable within Lucas when he has Offense / Defense Up, a bunch of healing skills, a shield move, and a counter. The only other offensive move he has is PK Flash and this is already self-explanatory. How do you implement a fun / good moveset when the only PSI abilities he has open to him is: Recovery Move / Counter / Shield. Tbh I don't want another Counter moves and personally don't find them interesting (and it won't really make him unique when 10 other characters have one), a Shielding mechanic COULD possibly be interesting to make him take reduced damage but I'm not sure how they'd implement that. The only interesting thing would replacing his down special and possibly implementing it to be like Wii Fit Trainer's down B, healing a little bit as well as giving him Offense Up or Defense Up (two in one and is unique to his character) but that's really all they could do to make him more unique.
Also assuming they followed the pattern of X teammate teaches Lucas how to do X move, well Duster is still a very support-like character and can only afflict ailments on enemies like smokebombs (decreases enemy accuracy), a snag beetle (makes enemy annoyed with sound), a staple (holds enemy down), a pendulum (enemy fall asleeps), a scary mask (decreases enemy offense), and a tickle stick (decreases enemy defense). I would like to see how they'd implement any of that at all to Lucas's moveset including Lucas never really striked me as a really strategic character anyways that would use tools such as these. Other possible move would PK Ground from Kumatora but I can just imagining it working exactly like Donkey's Kong down B so in a way it wouldn't be unique anyways unless it was his smash attack. Lastly the only thing aside from a Recovery Move that could make Lucas slightly more unique is to implement the rhythm combo mechanic from the game into his moveset similar to Marth's Dancing Blade and making you learn the timing of the move similar to the actual game but that would most likely mean PK Fire would be removed.
Like I really don't see how Sakurai could have made Lucas unique IMO aside from those two things I've mentioned and I'm pretty sure he wouldn't want to do something similar to Offense Up / Defense Up as the Neutral B considering Project M is already doing that and even than that's only one move. Even in Project M when they try to make the character more like their game, Lucas isn't much different and his PK Freeze is basically a better PK Fire that functions similarly but just looks different and has a different name.
Also I wouldn't even compare Lucas and Ness with Dr Mario and Mario. Dr Mario has a few differences from Mario while Lucas has a huge list of them, as said by @
elusiveTranscendent
, their jabs / tilt / smash (aside from f-smash which is similar but different properties), and aerials are different. Dr Mario has the same exact jabs / tilts / etc. aside from down air, and the major difference about his special moves is his down special while the rest are like Mario's but with different properties. I'm not trying to say Sakurai isn't lazy because I personally find it ridiculous how characters like Lucina weren't just included as alts and also how we still don't have costumes for characters and only recolors.
This so much. I wouldn't care that Lucina or Dark Pit were in this game if they were unique in terms of their normal attacks but they are just blatant clones with very minor differences.