Now that Melee vets returning is indeed confirmed, we just need Mewtwo to round it out so Dr. Mario's inclusion is justified. But that's for another topic.
Anyways, I can see him with his Melee moveset as the most reasonable choice. However, being an advocate of moveset diversity and uniqueness, I hope Sakurai and his designers expand upon his semi-clone flair:
-Pills are fine how they are. Although having nerfs, elemental damage, or ailments on certain pills would make a great addition.
-Give him a syringe pogo attack for his new Up Special. I can see it being more effective when it has a surface to boost off of. Maybe have multiple jumps after initial execution depending on charge to spice it up.
-Down Special staying as Mario Tornado is perfectly fine. To differentiate him from Luigi, make it's upward trajectory easier to angle, reduce its speed and traction when traveling sideways, and give it a property that greatly suspends his weight in the air.
-Mario already has enough custom variations to make an electric cape not special. Give him a defibrillator attack where he rubs it together to build static before extends its pads forwards and produced a mighty multihit attack (think Robin's Elthunder). Or in the case of a full charge, produce a short-ranged, transient projectile. I can see this being useful for spacing.
-Don't make his Final Smash a lame Mario Finale variation. We've already seen what Landmaster/Starstorm redundancy can do. How about incorporating the three Viruses from the games in some way?