Brief off the top of my head (I’ve only completed the 3ds version once, so my memories Haze. Really outta replay it though soon...)

-Song of double time now allows you to any hour in the current day instead of just the nearest half day (a huge improvement for sidequests, like at Romani ranch).

-you can now save at specific save spots without needing to restart your 3 day cycle.

-The fishing mini game was added. Not really that important but many minigames that reward mucho rupee’s in the origional now reward “fishing passes” instead. Rupees are slightly harder to farm as a result (though still everywhere).

-the bomber notebook is given as standard by the happy mask miyamoto now instead of obtained from the bomber kid.

-i might be wrong here but I can’t remember whether Sheika stones from the 3ds Ocarina remake are also in majoras mask. Never used them myself so I never noticed.

Honestly it’s a great remake. Easier than the origional for sure, but still serviceable and can give a first timer some trouble. However I still HIGHLY recommend it :)