What an interesting question! I guess one angle for you to think about is how Act V scene i resolves the action or not. Certainly Twelfth Night is a typical Shakesperian comedy in the sense that the action seems to get more and more complicated, spiralling out of control. Are we convinced with Shakespeare's resolution, or does he himself withhold a "happy ending" to a certain extent?
One way to begin would be to examine characters who, for one reason or another, are "left out" of the happy ending. The classic example is Malvolio, whose treatment strikes a note that jarrs in comparison to the gentle comedy of the rest of the play. He is definitely abused and his final words when he is brought out in front of the company strike a very disordant note compared to the celebrations that unfold:
I'll be revenged on the whole pack of you!
His treatment certainly points towards the dangers of excess, of taking revenge and petty squabbles too far, by featuring a man who is punished far more than his faults allow.
Another character to focus on is the treatment of Antonio and how he appears to be forgotten by his love, Sebastian. He, having only one love in Sebastian, is automatically excluded from the "inner circle" of marriages that feature the resolution. What is his fate? Where is his happy ending?
Thirdly, you could focus on the absence of Sir Toby Belch and Maria. The majority of Shakespeare plays end with multiple weddings that happen together. We know that they have left fearing censure and punishment for their actions in gulling and then goading Malvolio, but are we convinced that there ending will be a "happy" one? Sir Toby seems to be a chaotic character, resenting order and conformity. Will his marriage last or does their exclusion from the "happy ending" paint rather a sad future for them?
Lastly, and perhaps most importantly, consider the role of Feste and his final song, which seems to mock the "happy ending" that is juxtaposed next to it. His song points towards the harshness of life and reference to the "wind and the rain" seem to emphasise the unkindness of life. The twelfth night festival marked a period of great joy and mirth and excess, but it, like everything else, must come to an end, and the "wind and the rain" is perhaps the reality that we the audience are faced with as we leave this fairy tale play with its happy endings when all problems and mistakes are resolved. Perhaps we can identify more with the characters that are excluded from a happy ending rather than those that share in it.