I think this is a fundamentally flawed premise. Thing is that if we take the stated motivation as his plan, all he'd end up doing is creating heroic cripples... that likely have no powers. Thus be useless unless he is going to start turning people into cyborgs... or download them into homunculus and use them as 'AI's in the same method as Ilya was or something. Basically a super soldier program. This is so maguslike in thinking it would obliterate UBW from this Shiro entirely.
In short... the crossover fails due to 'Shiro' being an In Name Only original character. At that point, why bother making him named Shiro at all? Even if you changed Shiro to the point the online world was basically and extension of his reality marble that causes anyone inside it to become a hero or die so their soul fuels other peoples transformations in heroes. This would require impossible amounts of prana to make that function for years on end... If Gaia didn't smite him at all. If I abuse the idea I come see an insane, grail corrupted Shiro hijacking their connections and making anyone who died in game enter a secondary world that basically was Gantz. At that point they enter the real win or die scenario. Though I don't think the magus mentality Shiro can still be Shiro enough to be Shiro. So the crossover fails due to flawed mechanics rather than in concept.
OTOH, I think Shiro goes mad and ends up as the canon Gantz' in the ball Gantz. EMIYA ending up so desperate for as semi-competent minion he starts bringing in subjects at near complete random does fit. It would be so very much a rude gesture he'd commonly get if the useless, but unrekillable dog was his success story. You wouldn't even need to change anything in the plot until the point Gantz gets out of the ball, and then puts on pants and annoys the male lead by following him into combat and declares him Shiro's new plucky sidekick.
In short... the crossover fails due to 'Shiro' being an In Name Only original character. At that point, why bother making him named Shiro at all? Even if you changed Shiro to the point the online world was basically and extension of his reality marble that causes anyone inside it to become a hero or die so their soul fuels other peoples transformations in heroes. This would require impossible amounts of prana to make that function for years on end... If Gaia didn't smite him at all. If I abuse the idea I come see an insane, grail corrupted Shiro hijacking their connections and making anyone who died in game enter a secondary world that basically was Gantz. At that point they enter the real win or die scenario. Though I don't think the magus mentality Shiro can still be Shiro enough to be Shiro. So the crossover fails due to flawed mechanics rather than in concept.
OTOH, I think Shiro goes mad and ends up as the canon Gantz' in the ball Gantz. EMIYA ending up so desperate for as semi-competent minion he starts bringing in subjects at near complete random does fit. It would be so very much a rude gesture he'd commonly get if the useless, but unrekillable dog was his success story. You wouldn't even need to change anything in the plot until the point Gantz gets out of the ball, and then puts on pants and annoys the male lead by following him into combat and declares him Shiro's new plucky sidekick.