There is already an XP boost / challenges..but…it goes to player XP and not character XP.
They definitely need to ad some sort of challenge mode or tower. I like on MKX how you can change your fight card to boost certain XP/FXP or koins.
The best thing i know to do is a lot of counter breakers, shadow counters, ultras, combo breakers, pop instinct, and try not to whiff any attacks. Thats the best way to reach the highest XP