Bumping this thread, but I've just played through SMBNext: Halloween Special. Very fun one-world hack and the overworld theme is stuck in my head now...

In the sky bonus room of W1-1 (where Star Coin 1 is located), you can fall through the hole the vine goes through... to your death! I've lost four lives to this. This is annoying. Also, in W1-4, some visible blocks are intangible and some tangible blocks are invisible! Lol. And in W1-Castle, I think the Star Coin in the bouncy blocks room should've been Star Coin 1 (it is 2, so there are two Star Coins 2. Which probably means there's no Star Coin 1 at all. I couldn't find it anyway).

But enough negativity, on to the things I liked (I haven't 100%ed it yet (I have yet to find the secret exit of W1-Tower)): I really liked the gimmick of W1-Castle pre-castle: the initially-'collapsed' ground that 'repairs' itself if you go above it. And the blocks are placed the way Nintendo would place them, well done!