Well, its precisely this which does not make much sense anymore. Without some sort of an attack your dash dance is silly and doesn't mean squat. You can wait out Marth until he runs himself into a corner, then just straight up come in with some sort of aerial on his dash back. Or straight up chase him down on the dash back until it runs out of space on the stage.
I opened a fortune cookie today and it said this:
"One does not have to attack in order to be aggressive. The most effective aggression is often the most passive.
Like taking space while dashdance camping."
It all kind of comes down to your Dash Dance game, and by that I mean observing what the opponent is doing, how your spacing is, and if you can punish(approach) the enemy from where you are or for what he/she has done. Dash dancing/threatening with his range, like Bones0 said, is the most effective tool that Marth has in neutral.
If it helps, you can use 4 scenarios to kind of summarize the different neutral options and what to do against them:
Attack
1} Aggressive Offense
2} Passive Offense
3} Aggressive Defense
4} Passive Defense
Defend
1) Aggressive Offense
Fairly straight foward. This is the kind of "f*** it, i'm goin' in hard" style that sacrifices defense and playing safe to just go for all in attacks. This is also the easiest for Marth to deal with since he can pretty effectively block off approaches.
2) Passive Offense
The "cautious attack" kind of deal. A little harder for Marth to deal with, but this is the "apply careful pressure and stay on offense" kind of plan. This sort of strategy usually revolves around the opponent making the "correct" decision.
3) Aggressive Defense
Mostly revolves around allowing opponents to make the first move and punishing it, or forcing them to do something stupid/bad by encroaching on the edge of their space. Works most of the time if the opponent does go in first, but loses to getting out camped. This is where I think Marth thrives at.
4) Passive Defense
The complete opposite of the first, a case in which the opponent does not approach. allowing/forcing opponents to approach and then dealing with whatever is thrown at them. Ideally, Marth would be here, but he has no tools to
force his opponent to come to him.
An example playstyle of each (Tentative) would be:
1.Mango
2.Peepee's Falco
3.M2K
4.Armada (ish)/Fox vs Puff
Now, of the above cases, Marth beats out the offensive ones (assuming he plays it correctly). The last two are what Marth struggles with, due to the poor man's approach kit. TBH, I can't answer this
I just do not understand how he can force his opponent into those situations where his punish begins.
because I don't think there is a legitimate answer for it. The only thing I can think of would be aggressive DDing, but if you go too far in, its considered an empty approach (which usually goes just oh-so well). The next step would be something along the lines of spaced d-tilt poking or grabbing since those are his safest neutral options, but even then, if they block it/sidestep, you're in a nice combo position for them. That's why earlier I was saying the best offense you can really have in this case is to be tricky. 20XX is a lie, and no one can ever get there. That is, imho, the only reason all characters are still relatively viable. If you want proof, go look at M2K bop some scrubs with his Pichu, one of the worst characters in the game, and definitely worse than Marth. At the end of the day, if you wanna play Marth and win, you have to play your opponent, and remember that he/she is a human, not a computer. Marth might not be able to succeed in the perfect setting, but the perfect setting will never happen, and thus, I can still sport an awesome blue cape on my character.
Now, if you wanna talk about ways to trick your opponent to get in, we can discuss that all day long