This is how I play him (I use Fox/DDD/MK)
Remember the last days of Brawl? Top players would use MK's dash attack as their main approach option. Well, it's unchanged, and it opens up the game as much as dthrow does. To dash attack quickly without a c-stick, flick the circle pad so you've let go by the time you press the attack button. To manage spacing, walk or short hop around. Dashing/walking and powershielding helps deal with most projectiles.
Upb is an out of shield move, a juggling tool, and a KO move. One of the game's best moves. You can surprise with it, use it high up offstage and drift back to the ledge, wait out an airdodge or aerial and punish, or combo it out of dthrow/dashatk/uair. Remember you have slight horizontal control over each slash so you can land the second hit. It'll be very easy with a good controller and no lag in the WiiU.
Downb, combined with MK's 5 airjumps, lets you regain ground or get back from the ledge easily since it becomes invincible and invisible and it covers a lot of distance.
Nado is useful but very risky. It counters ledgeclimb, ledgejump, and hits people out of anything that involves whiffing attacks, not attacking and not blocking. Plus it can deal 20% even on small characters.
That's mostly it. My combo staples from dthrow are dash attack, dash upair, dash full hop fair or upb with or without dash. From dash attack, they're uptilt, uair or upb. From uptilt, they're utilt, uair or upb. From uair, they're uair or upb. If you have good enough execution to accurately follow the opponent's DI, you can deal around 50% from any series of follow-ups that lead to uair, and a series of airjump uairs into upb, but my experience (maybe because most of it's online) has proved that the safest thing is to ensure a hit by finishing the combo with upb as early as possible. Any of these combo options may lead to walking around in order to counter a response.
Luma has let me gimp Rosalina upwards (hilarious but true) by hitting me out of upb, so I airjump after Rosalina and upb her again offscreen for the KO. Megaman is probably susceptible to the same thing if you get a chance to spring off his Rush Coil. If an opponent, in a juggle attempt, jumps after you to high altitude and tries to land an aerial, you can kill them with airdodge into upb (or at least damage them).
I don't have an edgeguarding method yet; too many options (both MK's and his opponent's) to consider. I kind of wing it and think that eventually we'll gimp characters like Falcon, Lucina, Lil Mac etc. at least once per game (specially Mac, of course; they designed him to get gimped).
Remember to use dtilt (this one's quite good outside of shieldgrab range and stops dashgrab approaches well), utilt, and dsmash, and kill with usmash/fsmash (or whatever; just remember you have a lot of decent moves and you should use them). Usmash is one of the easiest kill moves to land so notice when they reach a % where it can kill. An observation of mine is that opponents usually become very easy to hit every time upb can kill them, because it's a very intimidating move that can kill very early (if you're in rage mode, they'll probably be terrified every time you put them in the air).
I think MK's main weakness are characters with KO power, as he has a great recovery but is very light. A secondary weakness would be projectiles (although I don't think unintimidating projectiles like Pit's, Sheik's and Falco's should cause too much trouble). Our worst match-up may be... Fox, I guess (at least that's what I can tell from my experience, which isn't too much to be honest). He combos and has KO power, has a very long range projectile, and his low edgeguarding ability doesn't really matter since MK's very strong against edgeguarding anyway.
I feel that our main edge would be against light characters that will die to upb at any moment, and characters with mediocre or worse recovery (that can -really- die at any moment).
