Oh, how would you mod it? :O
Sorry for late response, day job.
well first thing i would do is add a bit more to the 'easter egg' with the locker, probably press 'interact' and get a 'pulled in animation' and a window popping up with another 3-4 stage animation like your other title.
Next i would add in an extra character to choose from, perhaps unlocked through the vending machine (just to avoid adding too much more UI) most likely Misako.
Next i would add more enemies/scenes, start with one, add another, maybe one more before moving onto
Making more floors.
More outfits.
Then repeat the loop until i either get tired/bored/dragged into any number of other projects. (im currently renovating a Shuttle to an RV, and We are taking a break from our 5-10 year investment into a LOOOOOONG RPG passion project to work on a MUCH smaller VR Reverse Horror game. Just made our first level last night after work in fact, we are working on movement mechanics and the feel around that first.
But yea, that would be what i would do :D