So Victor describes the biochip as a 'ticking time-bomb' and that no one knows how long V will have before they die...
[Certain parts of this concept might only be possible with a robust set of modding tools... or be taken on outright by CDPR.]
As a novelty for people really looking to roleplay the V that they play, I think it would be appropriate for V to potentially die during a randomized Relic Malfunction. I think in general terms it would work something like this :
The player starts a new game or loads a save and if this countdown timer isn't present already, a randomized countdown timer (based on the in-game time) is started outside of a determined time frame (possibly set by the player in something like Native UI)... This way the player can know they will have a certain amount of time in game before the Relic really might kill them. Ideally this timer will persist between saves until it expires. From here the player goes on as normal and is not able to see the timer counting down. Should the player reach the timer running out, a Relic Malfunction effect overlay plays, V's health and stamina drain, V coughs up blood, and eventually collapses... Flatlined.
Perhaps the player could start to see the writing on the wall ahead of time with five or so relic malfunctions that drain growing percentages of V's health and stamina each time. (Currently, I believe that the relic malfunction affect normally only drains some amount of V's stamina.)
Maybe elements could be included to extend the timer like eating, drinking, and sleeping. Checks and balances would have to be considered so that a player could not sleep continuously or gorge themselves on food and drink regularly in order to artificially extend the timer... Nothing about the foods and beverages in Night City suggest that they wouldn't kill V on their own in excessive quantities. (Though I'm sure they are full of preservatives.)
I don't know how this part would work exactly, but I think that during the final relic malfunction death sequence (or maybe a few minutes before at the start of a secondary point of no return countdown) the game should make a save with the main timer expired and removed so that the player can reload and start over from that point with a new timer unless they want to roleplay their V and start a new game hoping for more time.
An alternate option to reloading a save or starting a new game after the countdown expires and the biochip flatlines V is that the player is given the option to have their character wake up where they were but they are physically transformed (or transforming?) into Johnny (impressive c%ck and all) and the player now gets to roleplay a rockerboy-gone-solo with an axe to grind... or amends to make... or both. Might have to unlock his iconic arm as a cyberware at this point to complete the effect... Maybe get a lead on it from another Adam Smasher minion in an additional Side Mission with Rogue and a few other top tier mercs. (For sure Adam Smasher would have kept it as a trophy when he got rid of his body. Continue gaining experience and abilities as before... Maybe fill in the sixth Attribute slot with a perk tree that is specific to Johnny or a Rockerboy character. (Now that the NPC civilians can be armed, maybe Johnny can incite them to violence and have them join him temporarily.)
Initiating any of the final confrontation sequences would have to be at the discretion of the player somehow since the normal in-game route to starting them would now be gone. I guess all of the final confrontation sequences except siding with Hanako or suicide (because, why?...) should be made available but the only sequences that would really make sense are the ones where Johnny would have been in control; by default the secret ending would probably be unlocked and initiated by walking into the Arasaka HQ building. Maybe just setup a phone call to Rogue or Panam and start a series of dialogues to begin their respective final confrontation sequences. The motivation for getting into Mikoshi would have to change to doing it out of spite for Arasaka with the goal being to get Alt access to free the stored engrams that have been interned there and finally destroy the program; all of this in lieu of V no longer be a factor in the story. All of the messages in the credits would still make sense as V just Faded Away... [ see what I did there :) ] After the credits finish, the player could continue on in the open world of Night City as a triumphant Johnny with Mikoshi destroyed and Arasaka collapsing from within since Yorinobu would have likely survived and continued what he had intended in The Devil ending narrative without 'losing himself'... assuming Hanako doesn't herself have him killed. Any unfinished missions/gigs would need references to V in dialogue would need to be changed to refer to Johnny in some way.
The end goal of this is to place a sense of urgency (if self imposed) on getting through all of the main story as V and possibly having to pick and choose what side content is pursued. If the 'continue as Johnny' option became a reality, it could also open a lot of alternative narrative options within the course of normal gameplay but I think that would take CDPR getting on board. In my opinion, it doesn't seem like the player as V should have enough time left (unless they are exceptionally fortunate) to do everything on the map in one playthrough.
I think this would pair well with NG+ if it ever comes; though it would probably would be best to give the player the option to skip The Heist and start up at the Dex meetup afterward. I don't think there is a lot of value in replaying the content before you have the biochip unless the city is opened up somewhat so that you can level your character more beforehand even though there is only one way to complete The Heist. Alternatively, give V the option to slot the biochip first and get Jackie to a ripper so that he can remain with you for your remaining time... Or have him keep the biochip and he slowly turns into Johnny after he dies. A lot of dialog would have to be added in order to just hear Jackie's side of the conversations with Johnny unless V and Jackie were somehow made to share Jackie's perception of Johnny. Maybe have Jackie die in the final confrontation sequence so that he becomes the NC legend that he so wanted to be and really earn his drink at the Afterlife.. Meanwhile V goes on as a living legend.
So... that got a bit out of hand.
Kinda started off as a mod idea and turned into a expansion pack.
In any case, maybe it will inspire someone with the know-how to implement it.
Just give credit for where the inspiration came from...