More things that we can Fsmash OOS for free!
Luigi's Default down B, unsure about the customs.
Ledge get-up attacks.
Default Metaknight Neutral B. Lol
Duck hunt Side B, but this is REALLY hard, you have to be pretty close to him already and the explosions can get in the way if they detonate it as soon as possible, but it appears to be possible, Ftilt will probably be generally better though just because it's easier.
By the way, since there was more talk about Down smash, I decided to look into it, I ran a test with a friend of mine using Marth, and had him use all of the standard ledge get-up options while I tried to punish him. At first I was using Up smash, and I realized that it seemed to be a good way to beat multiple options. However, it didn't do anything to Marth if he just hung out on the ledge and waited. Then, we took at look at Down smash.
So basically the idea here is that when Marth grabbed the ledge, I would start charging, then simply let go on reaction to any movement that I saw. It wasn't incredibly hard to do at all, even when I asked him to mix up his options and timing. The Down smash won against every option, including the ledge attack and ledge jump, and to make things even better, as long as you space it properly, it also punishes marth for just hanging on the ledge, so you can just let go the instant ledge invulnerability is gone.
Now obviously, this will probably lose to ledge hopped counter, and aerials, I'll do a bit more testing on that with various characters. However, that aside, this could probably still be useful as a surprise edgeguard, especially against anyone without a counter. What makes this even better, is that the spacing for this edgeguard also allows you to use grounded down b to the ledge, which could be used for ledge trumping or punishing recoveries with little or no ledge snap.
The spacing for this, is when you crouch at a ledge, Bowser's hand should stick out a bit, just his fingers, and you should be able to bomb the ledge from where you are standing and not die. The spacing is very important, too far in and you don't cover the idle option, too far out and you don't cover rolls. Also, in both cases you could try to bomb and either land on the stage for a free punish or SD for a free stock.
If anyone would like to do some of their own testing, either in the lab or in actual matches, and post results then please do that. I'll probably be putting all of this into the OP if it turns out to be as good as it looks.
EDIT: Looked into this a bit further! It appears that when Marth tries to jump from the ledge to the stage directly with an aerial or a counter then Down smash will stuff the ledge hop. However, he can still do a retreating Nair from the ledge to hit you or he can drop down and up b to hit with that as well. Bowser bomb and firebreath can cover retreating options though, and if the counter comes out in the air, grounded klaw can grab it if he's not too high, or you could just normally grab it I guess.
A few times, the aerials traded with the Down smash, but it usually just threw Marth back offstage, so the trade appears to be favorable.
That's about all for now I guess.