That sounds like something like an extra BG layer or tile layer of some sort

Underwater piranha plants aren't non-canon or anything. They appeared in SMBTLL and NSMBW.

I've made an interesting discovery about the "IF" activator and its enemy watching ability.

It isn't actually based on the enemy's location. Its based on what location the sprite was placed in, in the editor. The location is just used to specify which enemies are to be watched - it doesn't even need to cover the enemy, just sprite square. The enemy can walk out of the location, and the actor will still only switch the flag once the enemy has been defeated. This doesn't seem to include the enemy falling into a pit, but lava does count. The IF activator probably needs to be on screen for it to work properly.

Is this new? I'm pretty sure it works differently in NSMBW. Correct me if I'm wrong though.

Did you make a push request too?

* groans
* Newer DS
* NSMBe
* DS ROMs

are indeed things

You are joking right? Describe or screenshot it *in extreme detail*

Three more questions:
1. Why doesn't it have support for nybbles 1-4? That way you can use skawo's "Actor Spawning ? Block" to spawn actors that require events to be visible or to activate events
2. Could you add a despawning feature?
3. Where does the Actor Spawner get the sound/particle IDs?

Also, here is a start to a SFX table - on the wiki so anyone can edit (™)
http://nsmbhd.net/wiki/Actor_Spawner_SFX/

Actor 21
Nybble 5: Entrance
0:Default,
1:Pipe facing up,
2:Pipe facing down,
3:Pipe facing left,
4:Pipe facing right,
5:Door,
6:Same as 0?,
7:Ground Pound,
8:Same as 0?,
9: Swimming

Nybble 7: Unknown 7
0:Default,
1:Uses the floaty physics
2:Autojump on edge of block, Fast jump & fall, permanent jump until cancelled with g-pound,
3:Go up all the time,
7:Can't go up or down and other weird limits

Nybble 11; Character
0:Mario,
1:Luigi

Nybble 12: Unknown 12
0:Default,
1:Clone not present, Mario warps to world map,
2:Clone not present, freezes,
3:Clone not present, Game Over triggered,
4:Same as 3?,
5:Clone not present, freezes in a different way,
6:Same as 3 but with wonky camera?

You can already do that with a bunch of tilegod sprites. Also, it probably wouldn't be that hard to increase the capacities of the width and height settings.

I (and in the past, orangefox) have discovered some event IDs that cause weird stuff to happen. One of them is used: 0x3E, which causes a trail of blue coins to be left behind by Mario, but the others are unused. Some of them seem to cause some unused code to be activated, and I wondered if someone here who is good with ASM-type stuff could work out what these event IDs are actually doing.

http://nsmbhd.net/wiki/Guide_to_Events/

You say that like this is an original idea

Very good work. I don't really understand the "scrapped width" thing though, what is that setting about?

Also, have you done something with the actor sets? They are hopelessly outdated in the main fork (done by sprite rather than actor, old names (like "Pipe caterpillar guy" instead of "Squiggler", presumably missing stuff).

You are meant to put it at the bottom of the camera.

And/or because it is just easier to hardcode than to make flexible code.

If you just want invisible paths, you can make custom "track" tiles that are invisible or non-destroyable ground/decoration

Skawo probably put an off-screen solid object to prevent the mushroom from "running" away. I'm sure he said it can't run away, and that is the simplest way to prevent it from happening IMO.

Those pipes are seen in prerelease materials.
Hiccup knows these things.

No way to despawn them AFAIK.

In the "NSMBdex" sticky thread you can find instructions on how to do wrap-around properly. Applies to mvsl and singeplayer