So we've all had poor backfill experiences due to a number of reason. You're often thrown into a losing match (hence why the person left) and are already at a disadvantage.
To help give a fighting chance, I suggest the backfiller is given super energy proportional to how much of the match has been missed. This can be calculated off of time elapsed or current score or an algorithm combining both.
It sucks to load in with no super energy while the opposition all has there or are close.
Now this suggestion won't fix everything but I think it'd be a nice QoL change for all of us.
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Honestly the best implementation would be to divide the super bar as you said by the time in a match. 10 min match is the norm right. So break it down to 1/3 of super bar for every 3 mins that have passed in the match for any players that join.