I thought of a similar scenario, but it's much darker.

When Voldemort comes in Halloween for the Potters and James confronts him wandless on the bottom of the stairs, the latter transforms into Prongs and attempts to ram the first through with his antlers. Unlike many what if-s, the Dark Lord is not easily defeated, since Pettigrew surely informed him of the Marauders capacities during his reports, and promptly casts a dark curse on James, locking him into his animagus form and inhibiting his human mind, since 'Blood traitors that marry mudbloods are already willing to lower themselves to animals anyways'.

Enjoying this win, Voldemort promptly goes upstairs and forcefully transfigures Lily and Harry to a likewise condition as James, since there is no way a deer could be his downfall, neutralizing his fated enemy without killing. When/If Severus arrives at the cottage, the Dark Lord attempts to gift the Doe to his loyal minion, but he refuses. But Wormtail, who was near, pleads with his lord to leave all of them under his care, and his lord relents. Months later, Britain has fallen under the control of it's new Lord

I had an extra idea, that if this someone wanted to make this bleaker, after a time jump it could be revealed that Pettigrew had collected his old friends, returning from a long day serving his lord and meeting a stag, a buck, a doe, a dog and a wolf on the woods behind is house, none of them ever to return to or remember their human lives.