I think it is possible for the maidens to all have a happy ending as long as Rozen changes the requirements for Alice which he did in the end of S2.
Hina - Though Jun, Shinku, Nori and even Sui, I think she reached her happy ending. It would be better of course if she was alive to enjoy it
. But she I believe achieved what she wanted which was not to be alone.
Shinku - Is much closer to reaching hers now she knows that killing her sisters is not the only way to become Alice. All she needs to do now is to save Hina and Sou and convince her sisters that killing is not the way forward.
Sui - Is kinda the same as Hina's, but she is a little more complex in that she wants to be loved too (centre of attention). For her it is to be together happy with her sisters and meeting father is defiantly a lower priority, what a gal!
Sou - Hers is making Rozen happy, so all the bugger needs to do is go "I'm really happy with just the fact I know that my creations are happy". But since the requirements have changed she no longer needs to do the Alice game.
Gin - To save Megu which has even over ridden her desire to be Alice. This here I believe makes Gin such a wonderful character. Her love from Overture and S1 imho were a very psychotic form of love. But meeting Megu I think has let her character blossom and given her a real heart that is starting to understand that love is a 2 way process. This can be achieve I think by either Megu realising that she wants to live or if her illness is terminal, some N-field funky magic. Personally rather then being stuck in a hospital hooked up to a machine I wouldn't mind spending eternity in a nice n-field with the person I care about the most.
Kana - Acknowledgement by the other maidens (which she is starting to get at the end of S2) and to keep her medium happy, which Shinku et al granted her. Now all that needs to happen is for them to establish the Rozen Maiden fan club and get a club house (bastard bunnies not allowed)
Bara - For her numpty of an creator to get his head out of his arse and realise that his own creations are beautiful in their own right and to stop trying to compete against his master.
Kira - Wild card right now....