Mark Miller the creator of the Kick-Ass comic hits the sequel is to be about bad kid reacting to super heros consequently becomes super villains and Hit-Girl try to lead a normal life. it will be a bit like A History of Violence meets A Clockwork Orange. Read about what Miller said here.
This is what i am guessing will happen in the Kick-Ass trilogy.
Hit-Girl(HG) trains Kick-Ass (KA)
HG teach KA “the code” to not use violence against the innocence, the code is a less rigid version of Dexter‘s code
Marcus tries to normalize HG’s life
HG pretends to be normal school girls.
HG develop further friendship with KA, Katie and other kids in school.
While living with Marcus, HG gain access to Police data network and begun searching for the perpetrator that framed Big Daddy with little success.
Red Mist (RM) stops coming to school to runs his dead fathers crime organization.
Super villain (SV) emerges and scourges over the neighborhood.
RM wants to avenge his dad.
RM direct Gigante to find HG and KA.
RM makes contact with SVs and provide material support.
RM uses SVs to draw out retired KA and HG.
RM did not reveal KA and HG’s true identity to SVs.
RM find out who KA and HG’s real identity and direct SVs to terrorize the school KA and HG’ goes to.
Katie was raped in front of Dave Lizewski by the super villains.
RM reveals to HG that her guardian Marcus Williams took part in framing Big Daddy.
Marcus Williams is deeply guilty and sacrificed himself to save HG and KS.
HG and KA without weapons was chase by SVs into hiding.
SVs continue to terrorized the neighborhood, killing many kids taunting HG and KS.
HG and KA goes to one of their safe houses and dust off their costume.
HG and KA fights SVs in school and kills them with the help of other school kids.
RM goes into hiding and gather his strength.
HG moves in with KA.
end of Kick-Ass2.
with new clues, HG begin to find out who was the perpetrator behind the framing of Big Daddy.
Wanting revenge HG disguise her self as a new SV and secretly starts killing the innocent love ones (many of them kids from the school) of perpetrators (many of them polices) and eventually the perpetrators themself
Before HG kills the perpetrators, she tortures to find out where RM is.
KA investigate seemingly unmotivated killing and later discover it was HG behind the killing and try to stop her.
With HG raising hell over the police station, crime grows.
Kick-Ass3 ends with death of many corrupt police and their innocent love ones by HG’s hand.
Lots of action with KA fighting of crime while Police station is parallelized by HG.
KA discover the motivation and connection behind the killing and start looking for RM thinking that how he can find HG.
As HG finds RM, and KA appears and try to convince her to stop.
HG kills RM then herself. .
KA realize that superhero is no fun and try to return to normal life
KA trilogy ends.
If KA series is to continue then the alternative ending is HG did not kill herself but when into hiding.