I would think the only way they could make a winter-ball-type situation happen for gridiron football is to somehow put the clamps on the more injury-yielding contact, even beyond kickoffs, and emphasize skills development, which might help preserve some bodies but might also make it such a different game that the learnings might not be well translatable to the League. I think they’d also have to limit it to a six-game season, plus two playoff games max, ending in mid-May. Twelve games of competitive football from March into June, with no weeks off and League training camps opening in July, would be too much even for skill position players. And beyond that, how do they justify trying to season their best talent in spring football only to lose them to a career-ending injury?
They’d also have to make UFL rules identical to NFL rules. Can you imagine the uproar after an NFL game is lost because player couldn’t remember the rule for whatever is different in the fall versus in the spring?