Serial I think we can all agree, if used properly, is incredibly broken. The simple fact that you can theoretically drop 24 aliens in 1 turn is completely absurd. Making it a 7-turn cooldown really does nothing to balance this perk, you could make it a 70-turn cooldown and the only difference would be how long you spend waiting for it on waterworld
I was listening to JoINrbs mention it briefly in a recent video he put out. He highlighted the point that Serial is only really OP if you know how to use it effectively. The other options, for less skilled players may be more attractive simply because they 'just work' and don't require a lot of tactical knowledge and setup to use effectively. And to this I agree. But the part I don't agree with is that balancing this perk from it's current incarnation is hard.
In a time long ago, when I swore we kicked the aliens ass, we had a lovely little perk called In The Zone. It was just like Serial with 3 differences. It required a flanked/exposed target, each kill reduced the damage and crit chance of the next shot and there were no autoloaders! This made it still quite powerful, but was not always an autopick. There was a place for double tap, though I'm not sure mayhem seen much love.
I could imagine Serial being changed to something like this.
- 5-turn Cooldown
- -1 Dmg, -10% crit after every successful kill.
The dmg/crit debuff was what really put some semblance of balance into ITZ. The flanking part mostly made it kind of gimmicky and simply made it reliant on cover destruction to be useful at all. You normally want that anyway as it makes for more 100% shots. The dmg/crit debuff would be plenty to give it some balance.