@ Demi
Yeah, the eerie similarity is uncanny, the only difference between AU!Ichika and Fem!Ichika is that AU!Ichika is more serious (being a child soldier and all) while Fem!Ichika is slightly meek. The only problem is: What could be done to stop canon harem from falling for AU!Ichika? My sis is pulling her hair due to this.
@ Eras
The choice is two: make it little serious or just make it extremely funny.
If you make it a little serious, Future Kazuma can't stay since that means original Kazuma will disappear in the future (return back in time). Otherwise, just roll with the most heartwarming option.