Hm, other than what was already mentioned, I'd say some important tech to learn:
1. quick uairs. this is vital. get a handicapped bowser to the top platform of yoshis and perform uairs. make sure they hit every time or you are going late.
2. utilt and uair transition. handicapped bowser again but on the lower platforms, but use different stages so you understand timings. test yourself on stadium, battlefield, yoshis, and dreamland (constant uairs here). alternate utilt and uair in various patterns to make it so that Bowser is basically always in hitstun.
3. shield drop aerial walls. run, shield drop, double jump, aerials (double fair, fair->uair etc). Generally work on shield drops. Shield dropped wavelands, shield drop->normal land->shffl (or EMPTY hop, amazing mindgame), etc.
4. Run-off fastfalls on platforms. Make sure you can run off a platform and fastfall 100% of the time. After you get that, mix in an aerial, a double jumped aerial, a waveland, or whatever you want. Never forget to do mixups after a normal land too; just land after a run-off and then try to do things (say, a shffl) immediately after landing lag.
5. tilt transition timings. Use tilts as transitions during your SHFFLs and ground attacks. SHFFL something and tilt as fast as you can. especially important for utilt.
6. dtilt IASA mastery. dtilt, hold for walk to trigger IASA, and then transition into whatever you want (wavedash, another dtilt, aerials, whatever).
7. learn to dash after everything. helps in the fox matchup. use and aerial? dash. ftilt? dash. land on a platform? dash.
8. dash cancelling with crouch. learn the extent of marths dashdance (its LONG) and when you have to crouch to cancel momentum. the use the crouch to do various mixups. dash up to the opponent, crouch, dash forward, pivot grab (grab them from behind). Lots of uses for this one; can help you become a lot faster.
9. wavelands on platforms. just practice various sized wavelands (not just super long and in place, every size) on all stages. Make sure you can make waveland loops; like, waveland on platform, run-off fastfall off, waveland backwards, jump, waveland back on platform. and then add aerials in there lol. watch some spaceballs samus vids to see how cool this can be...waveland platform approaches are legit; work them into your game.
10. ledge wavelands. waveland forward; waveland in place (RIGHT next to the ledge); waveland back (for bait), fastfall, jump, aerial; waveland back, double jump, land on low platform, etc.
11. ledge aerials. These are obvious but its important to make sure you get them out as fast as possible. make sure you can make the first hit of the ledgehop nair come out when you are parallel with the stage (don't be ON the stage by the time its out). get your uairs, dairs, and fairs off ledge fast too. be able to float back dair (so that you have to up-B) and survive; this is fantastic vs. Peach (a normal ledgehop dair doesn't reach sometimes). ledgehop dair firefoxing CPUs to get the timing right.
12. empty hop follow ups. empty hop in range of your fair (this is generally when you want to empty hop), fastfall, and follow with tomahawk grab, dash back pivot fsmash (if you are pro), waveland (very little utility), etc.
13. oos options. This is really hard to practice on your own but try your best. hold shield and use a shield option. maybe use a computer to hit you with something and dair it or something. this is best learned in matches. getting a friend to laser you so you can wd oos through laser pressure (shortened wavedashes) is vital. also approach with shield (like dash->shield or wd->shield) and quick oos option...great v. Puff (oos fair ***** after a bair). shield drop oos on platforms after hitstun is pretty cool if you can learn it.
14. quickness off wavedashes. wavedash forward, pivot, dair. things like that...make sure there is no stall time after your wavedash. be precise with that **** too...make enemy targets at weird distances and see if you can hit with the tipper dair every time.
15. as ppl said, practice chaingrabbing. use training mode's "escape" CPU setting to work on different DIs. lvl 4 CPUs also DI relatively OK. include odd chaingrabs (fthrow on falco early percent), chains on platforms (if they DI away from you on the edge of the platform, FF uair runoff and regrab -- TRUST ME on the this, like no marths do it), and if you have a friend, techchase follows. try not to pummel (ever), so you have greater % coverage in practice....also, pummeling at low % is a bad habit in general. dthrow dtilt traps (dthrow, dtilt, pivot grab when near the edge), edgeguard traps (dthrow, wd off, fair, up-b), etc. Make sure to follow up with an edgeguard offstage to finish all combos. don't get a habit of standing on stage and countering/fsmashing.
16. shield to stop momentum before aerials. Something I am trying to get used to. Basically shield right before you jump so you go straight up. Great for uair chains (a lot of times your momentum sends you past your opponent when you don't do this and therefore makes it really hard for follow up).
17. Pivot aerial walls. another thing that i have been trying to add. basically make standard marth aerial walls (double fair, fair dj double fair, autocancel nair, etc), dash back or forward quick, and remake wall. try different distances and make it fast. with shielding, the wall stays stand-still. should be great v. Puff.
18. pivot fsmashes. watch AustinRC vids if you don't think this is useful. Get the spacing to it down in various situations. Tipper it every time (remember, you really only want to do it on stationary targets; so after you bait a shield with an empty hop or a sidestep with dashdance, etc). work on pivots in general but this one is great.
19. uair routines on different stages. This one is a bit hard to explain but watch a lot of M2K vids when his opponent is on the top platform and its easier to understand. Basically work on floating between the two lower platforms to uair the top one and then dj to hit it again. Different methods are required for different stages, but make sure you can trap someone on the top platform as easily as you can on the lower platforms.
20. That crazy Taj movement ****. I have no idea how he does it so don't ask me. Its some reverse stickywalk moonwalk wavedash shenanigans that i don't even understand the hand movements for. it looks mad sexy tho.
21. Quick methods to grab the ledge. Get goooood at this. Very important. run, crouch, dd pivot, wavedash, fastfall, grab ledge, do ledgehop aerial is a good routine. you can do a similar routine with autocanceled nairs added in (M2K does this in the lolmaster friendlies somewhere). make sure you are good at grabbing the ledge crazy quick after aerials, grabs, etc. fthrow a sheik at mid percent and do the quick DD ledgegrab before she can jump to grab ledge...this is MAD good in matches. practice PCing too I guess (using Marths walk and then pivoting near ledge) but thats usually worse than just using bair as a turnaround or this DD FF method. If you are crazy good you can practice pivot runoff fastfalls off dashes but I can only do those 10% of the time; I don't think they are humanly possible tbh. They'd be beastly to continue platform combos if they were :/
I could go on I suppose but this is a ton of things to work on. I wrote this for myself as much as you...I need to work on this stuff too lol.
Besides, I ended on number 21. Which ya know, is Tim Duncan's number.