here's what i'm thinking. this is purely all my ideas and opinions, nothing Jay has influence towards...
If Jay Kills Off...A member of Dynamo 5-It would change the dynamic of the comic book. the focal point was around the five illegitamate children Maddie brought into the fold. Killing off one of them would "invite" another character, like Synergy or even li'l Firebird, or an undisclosed child into joining which still messes with the chemistry of the group. definitely moreso, if Synergy joins.
MaddieIf Jay would kill off Maddie then that would shake the foundation of the team; she is literally the keystone of the group. Without her, the team would dissolve- like what happened when Brain psyche-zapped her. Either the team dissolves and the series ends, the team dissolves and Auggie/Sandy brings them into the fold of FLAGG, or upon Maddie'd death the team is forced to join FLAGG.
Crysalis/SynergyWe haven't seen Crysalis in a while, but Synergy we last saw at the end of the "Sins of our Fathers" arc playing hero. Two things i see happening if Synergy were to die- Crysalis going on a destructive tirade, and an investigation of which the results show that Synergy was killed off by some inner-working of the gov't- bringing serious questioning toward why they had an anti-Dynamo raygun, and a Cap'n Dynamo puppet-clone... and possibly giving Maddie more reasons to keep the kids seperate from FLAGG.
Auggie/SandyAuggie's death, i can see driving Maddie deeper into keeping the kids away from FLAGG as her only trustworthy contact within the organization- bringing up the question: who was behind his death?
Sandy, though, we know him to being a puppet of some shadow organization. His death could probably bring about the big revelation of the big string-tugger.
FirebirdsKilling off one of the Firebirds would have a different effect, considering who'd die. Momma Firebird already holds animosty towards Maddie and the injuries li'l firebird sustainted in the SotF arc brought more. If Li'l Firebird were to die, that would drive Hector further over the edge [maybe even to steal the Strong Suit and becoming a villain

] and drive a final blow between Momma and Maddie, possibly even enough the Momma Firebird to join the FLAGG team if Li'l Firebird's death was questionable; especially since her powers are being suppressed.
If Momma Firebird were to die, Li'l Firebird may join D5 for lack of any other sense of direction.
**Regardless of who'd die, Jay may finally confirm whether or not if Li'l Firebird was a Dynamic Child; making Hector dating his half-sister very creepy.
War Chest
If Jay kills off War Chest, compared to all the other Powers we've encountered, her death would have the most impact- as sheer
shock factor. [[Imagine Jay, in his dark apartment, sitting in an overstuffed chair, stroking his cat, laughing maniacally "They never saw it coming! Bwa-ha-ha-ha!"]] We don't know much about the over villains and other heros to make them significant if they were to die. the Teaser at the end of the SotF arc, where some Aliens were hunting Voltage, our only question is- Why are they hunting Voltage? not "Oh no! Voltage is gonna die! Don't do it, Jay!!" War chest is an undisputed fan-favorite, her death would bring people to tears. If Flagstone or Brawn died, we wouldn't even shed a tear.
Those are my views on who'd die and how it would effect the book.