Though probably a bit late now, thought I would share my input.
What I do in Neutral with Marth is a combination Ftilts, Dtilts, Extended Dash Dances, Fairs, Nairs and Dancing Blades
Extended Dash Dancing is good in Neurtal for movement. Ftilt is good when pivoted. You can also follow this up with a quick Dtilt, or a SH Fair to maybe catch an opponent trying to punish Ftilt. It is a lot safer to retreat if you are going with Fair, unless you full hop it, which brings me to my next point.
I quite often do full hop Fair into Nair, as this allows you to hit at just the right time on hit 2. It is also very easy to tipper, and is the strongest Nair in the game when tippered. I have gotten kills this way from Neutral in the percent range of 90% - 110%. Another thing I like to do is condition my opponent into getting used to this. Then, every so often, do a full hop Fair, but instead of doing Nair, cross up their shield and land behind them, and do a Dancing Blade. This works wonders for me.
The best part about this is if you can manage to land Dancing Blade Down 4. Say the opponent is in the middle of the stage, you do this technique, and land the Down 4 variant. The opponent at low to mid high percents will be pushed towards the edge of the stage, and you essentially have the opponent trapped. What you should do here is stand just out of range of their get up attack, and react to their get up. If they roll away, they will go towards the ledge, allowing you another Dancing Blade. If they get up or get up attack, wait for them to do this and pick your poison. Even roll towards you is not safe because 9 times out of 10, they will end up behind you, where you can do another Dancing Blade etc. Even if they tech it, it can still be punished if your reactions are quick enough. Done correctly, you can get upwards of 30% damage just off of this and reacting correctly to the opponent's get up.
Of course if you land a grab in Neutral, Dthrow to Uair/Bair, or Fthrow at 0% to Fair works on most characters, but the options above I personally find more useful. Occasionally throwing a Jab 1 out into the mix occasionally can work too if you catch somebody in it.
I hope this helped, and if anyone wants to disagree or input on this, be sure to. I want to learn as much about Marth as possible too.