Wonderboy in Monster World was what it was, and it was pretty good too.  To make this idea work, what needs to be done is find out where the game loads each area's data and find a way to import it from an overworld tile transition, to a screen to screen transition.  Since graphics are not updated when in a side view area, that means a new routine that would need to be written that would mimic the overworld transition pause.  The problem is that there really isn't much free space left, at least in the main bank.  This game has routines that are re-written in several places in the rom just so a bankswap would work without it all depending on the main bank.  That means a table would need to exist 5 times in the rom, and of course the fun part is that the Great Palace doesn't quite synch up with the other banks.  Good news is a table would only need to be 64 bytes, or else it could be more limited and be like an entry point type of data where only the rooms with a transition would hold data.

Another thing is what to do with the overworld items,  The flute, the boots, the raft, and hammer are all overworld specific.  They would need to be changed.  The problem does seem kind of ambitious, but would really make the game bigger than it currently seems.

Maybe it would be easier just to make an open source cross-platform sidescrolling game maker. Many many years ago someone in the Super Metroid fan community was making one. And there was a "Zelda 2 Classic" project that kept getting abandoned, too.

Also Wonderboy III the Dragon's Trap is another game like what is being discussed.