How about a cross between the Lightning Round & PvP format.
For example a PvP where you can only enter the event once & push your score as high as you can in one run. Once you leave this game mode that’s it - your score is set.
This would encourage roster diversity which the current mechanic does not - would also make a nice change not to fight X-force every single battle.
Maybe run for a 24 hour period (with a time out option to stop peeps leaving application open for healing).