I think Yugioh would've been a lot more interesting if everything wasn't designed from a one-player perspective. Having more cards affect both players would be a lot more interesting. It's one of the things from the anime that I adored with shit like Flute of Summoning Dragons summoning Dragons from Yugi's hand too.
That, and it makes logical sense for stuff like Field Spells to function on both ends. Mignon boosting only YOUR Yummys for EVERY LIGHT Beast on the field is dumb. You made the field the domain of Yummy, why would an opponent's Yummy deck not benefit from that?
It would also add an interesting twist to mirror matches as your cards are benefitting you and your opponent simultaneously so you may be tempted to use less of them or substitute them with less-specific cards to avoid the crossover.