Saw this on twitter, so will reflect my thoughts from there here: I think this would actually fit really well with the Dark Souls formula and several of its underlying themes.
Confronting the latter first, the idea of being a meek human before whatever hellish monsters FROM might cook up would be especially emphasized if you are literally an average person. Imagine picking starting classes with stats and 'skills' that are largely irrelevant to the game - salary man, shop keeper, commuter - because almost none of it matters before the apparent end of the world. Remaining vulnerable to damage because body armour simply is not common in modern, urban life, nor can you simply 'level up' some essence of your being to make bare flesh deflect steel.
In terms of the actual formula, well there's the potential ways of playing with the elements that make some vague sense in a created fantasy world where you're not even properly human, and much less sense in a nominally mundane setting. Then there's the fact that Tokyo's dense, sprawling, and at times quite vertical architecture would lend itself to a quite varied but interconnected level design not unlike the original Dark Soul. Want your 'dungeons' like the depths? Tokyo subway (which could also double as a fast travel system). Desire an ascension like how you went up the undead burg to Anor Londo? Start in the subway, get onto the streets of tokyo, and then fight your way to some high rise apartment or office building. Alleyways, backdoors, bridges, roadblocks - all of these and more could be used to enable the Metroidvania-esque shortcuts that helped make Dark Souls' level design so memorable.
As mentioned, on the weapons front you could do something similar to Dead Rising where a lot of mundane objects have to be used creatively, as Japan doesn't have widespread weapon usage. Finding a policeman's sidearm or someone's antique katana would be massive moments in that context, while realising someone else has a gun becomes a 'holy shit' moment as one has to run for cover.
It's got potential, and now I want this game.