Fireball as an approach option is pretty much out the window. The inability to fastfall them, buffer a dash after them, or do ANYTHING out of them makes them much less versatile than they were in Melee. Outside of general long-range spamming, there's not that much to use them for.
As far as I can tell--and this is after 6 hours of steady playing--Mario's approach options are either less than or equal to his approach options in Brawl. Considering how shield camping and rolling essentially dominate the gameplay (at least for now), long duration/low lag attacks are your best bet (forcing a roll and/or deterring shield grabbing/OOS attacks), which mean that d-air and sex kick are probably what you should be relying on.
Personally, I don't feel too comfortable with sex kick because it seems slow. But that's just me. I use it a lot less than I did in Melee just because d-air is a far superior aerial to be spamming at this point.
Otherwise, the b-air is pretty godly in terms of spacing/approaching. Using the run-and-turn-around-instant-b-air technique (whatever that's called) in conjunction with DI lets you space pretty well, and DI lets you drift away from the opponent, making shield grabbing more difficult. Plus it has fast startup and it autocancels, so it's definitely spammable.
So basically, shield-dashing and buffering landing dashes (for more precise movement), b-airs for spacing/approach, and d-airs for approach (possibly with some limited sex kick) are what I found to be particularly effective.
