Individual multipliers need just one more change
Compared to the old system, individual multis have been a blessing, by rewarding a good guess. The problem is, they also reward bad guesses, with the same amount of multiplier.
Both players can go wrong continent, but one will always be closer and get rewarded with the +0.5x (or +1.0x) multi. On the other hand, both players can make an excellent region guess with and... still get rewarded with the same multi increase. This can feel extra unfair in small countries or cities, espeically in turbo mode where scanning isn't that much of an option. It comes down to a coinflip.
The simple change that needs to happen is to increase multipliers based on the players' score rather than who is closer. That way, the player with more consistent guesses will still have a higher multiplier, while the other player will still get rewarded for an almost equally good guess to keep a fighting chance. And nobody gets rewarded on the wrong continent.
An alternative to this system would be to add some lenience to the current system. Say, if both players are within ~5% of their score, both get the multiplier increase.
I feel like this the one change that would make the game as fair as it gets.