Idea inspired by Fate/strange fake: Just because the Great Grail of Fuyuki was destroyed/dismantled after the Fifth War, doesn't mean everyone is going to give up on seeking the power it can grant. After the war, a number of attempts are made to revive the war, some involving the von Einzberns, some not, and perhaps a few attempts made in the original location, Fuyuki, much to Rin's disgust and offense.
Like Fate/sf, they're mostly (or all) going to be flawed copies, missing and/or distorting some aspects; for example, Servants being summoned into the Masters' bodies, leaving the Masters as passengers, at best, some or all of the Servants being False Servants (that is to say, local wraiths and other spirits taking the forms and approximate powers of fictional characters), or Rider Alexander the Great being summoned with EX rank Independent Action (awesome, but not something his Master would necessarily want - perhaps if that war hadn't been in northern Greece, it wouldn't have happened). Another example that popped into my head is Archer EMIYA being summoned, taking a look at what's going on, and then using Rule Breaker to free himself from his Master, and calling in the Magus Association to kill the people running this War before they can Break the Masquerade or bring the Counter Guardians down on the city from shear carelessness and idiocy.
The 'False Servants' variant could basically allow any crossover of characters that would otherwise make little to no sense in the Nasuverse, as long as you can translate their abilities into something that makes sense as a creation of a false Grail.
Like Fate/sf, they're mostly (or all) going to be flawed copies, missing and/or distorting some aspects; for example, Servants being summoned into the Masters' bodies, leaving the Masters as passengers, at best, some or all of the Servants being False Servants (that is to say, local wraiths and other spirits taking the forms and approximate powers of fictional characters), or Rider Alexander the Great being summoned with EX rank Independent Action (awesome, but not something his Master would necessarily want - perhaps if that war hadn't been in northern Greece, it wouldn't have happened). Another example that popped into my head is Archer EMIYA being summoned, taking a look at what's going on, and then using Rule Breaker to free himself from his Master, and calling in the Magus Association to kill the people running this War before they can Break the Masquerade or bring the Counter Guardians down on the city from shear carelessness and idiocy.
The 'False Servants' variant could basically allow any crossover of characters that would otherwise make little to no sense in the Nasuverse, as long as you can translate their abilities into something that makes sense as a creation of a false Grail.