[Thirteen 1355]

Regarding Mega Man 3, would there be any way to add more levels to the game? I'd like to add four Doc Robot stages, representing the four remaining stages that aren't revisited normally. I'd spread the mid-point Doc Robots among these four new stages.

If this isn't possible, I guess I'll just recreate these levels in the existing Robot Master stages.

[FAST6191]

Three main options I would explore here, tempered by what the ROM ends up being (I have not looked into these games that much)

1) It works as per any other level hack and will only break when you get to 255 levels or something silly.

2) Internal level numbering can be annoying but you should be able to add an extra flag and with that change all the pointers so you essentially have a second (or third as the case may be) set of levels.

3) You do the split game thing (think pokemon and its clones, or the Zelda Oracles titles, or maybe Sonic 3 and Sonic and Knuckles even if that from a technical perspective is not an ideal comparison) but for megaman. You should even be able to generate a password such that you have any relevant pickups/weapons to go between the different versions.

From what I have seen of this series I doubt 1) will be the case so that means 3) will probably be the easiest, especially if you can get the password "transfer" done within existing parameters. Less ideal but you could possibly also try transferring a savestate between the games (and maybe having a cheat to reset "completed" levels if that won't mess with weapons).
If you are willing to put the effort in and can handle any space, mapper/mbc issues then 2) is the most "correct" method if we are doing the usual pursuit of no funny stuff/press patch and load and works in hardware or accurate emulator.

[kuja killer]

Only if you use mm 3 improvement, it's possible. Not possible with original mm3 rom.

each level is 2000 bytes minimum, 4 more levels requires 8000 bytes of programming space, which is totally avaiable no problem on mm3 improvement (rom is expanded to 80,000)

But there's a whole crap load more work involved with the whole level order numbering thing, and many other things, like fast6191 said...

[Thirteen 1355]

Interesting. I intended to make two patches, one for regular, one for Improvement version (except when the regular one would be applicable to Improvement as well), but if this is the case, then I'll use Improvement as a base (the preferred way to play anyway).

Would any newly added level even be visible in a level editor, though? I just thought of that.

[FAST6191]

I don't know about the level editor in question and what it might change but a properly programmed level editor should be able to load its own changes. Start messing with tools that colour a bit more outside the lines and you could trouble a lesser editor.

Also if this is to be the case I would probably start from the savestate or password approach. For the savestate then I doubt there is any level preloading going on for something this old so you could take a savestate after you "finished" the original game and are at the level select screen, take said savestate to the hacked version and essentially have your progress carry on from there -- if you are only doing levels then it should work well enough here. This should then allow you to polish out the level design and after that you can contemplate fine tuning saves, or seeing if you can cram things into the ROM.

[Thirteen 1355]

Yeah, that's the approach I'm gonna take. Would go with two 'hacked' versions to accommodate for the spreading of Doc Robots over all the stages (1x8 stages instead of 2x4 stages).

In that case I'd simply start removing the midway Doc Robots in their stages in one version, and edit the regular Robot Master stages that don't have a Doc Robot variant for the other version. Then I'd need to make these four stages accessible once the Doc Robots appear, so you'd have to switch ROMs/patches when they do.

EDIT

Oh, of course I could also just switch between an unchanged/changed ROM, if you're gonna do all the Doc Robot stages on the changed ROM.

EDIT

Doing it with two ROMs seems unappealing to me. I'd like to know if maybe I could split up the two halves of Doc Robot stages, or even change them into final stages so I have more freedom in deciding their theming.