So now Tommen (who's had all of 4 lines in the past 3 years) is technically King. (even though he's just another one of Jamie's bastards) Sets up some interesting dynamics. By all accounts, he's the exact opposite of Joffrey (not a ####) and will probably be easier for Cersei to control. However, he also seems to like Tyrion. So is Tyrion now in a better position? (assuming he beats the rap for allegedly killing Joffrey) Kinda hard to tell.
Not sure what Lady Tyrell has to gain from killing Joffrey. Yeah, Margery married him, but without any kids, you'd think it would be easy for the Lannisters to just push the Tyrells to the side.