I have to agree with Seim, I really dislike RoA on Annie or most casters in general. I prefer to rush Rabadon's as it makes the rest of your game just as strong as your early. Also, I've never had trouble farming with her past the 30+ minute mark and her ratios got updated so she no longer flops off past 30 minutes, you might want that updated.
Molten shield at level 5 is debtable. If I'm in a solo lane I'll skip it till 1x, in a duo I'll get it at four/five.
You can also put DFG in there for luxury, it can be quite amusing melting tanks along with squishies.
I'm not sure about your choice of maxing Cassiopeia's Noxious Blast first, I might do so if I was in a solo lane, but the potential dps from maxed Twing Fangs outways the increased damage and farmability of Noxious Blast. I also want to try an item build that is something along the lines of:
Regrowth/Faerie Charm > Philosopher Stone > Boots > Catalyst > Rylai's > Rabadon's > RoA/Banshee > Hextech > Moonflair or some such.
Boots of swiftness are also very potent on her as the increased movement speed from them and her Q procs and eventually Rylai's allow for amazing mobility.
I wish you the best of luck in your endeavour; it's an impressive and great idea.