Last Chance - Help ER AC

Hi guys, 

I posted here before about failing one of my AC (super day) at Cap/Fidelity/T-rowe price and was feeling demoralised and my only chance was to go IBD for the moment as I already had an offer.  I recently got invited for a final round at Fidelity on short notice after netowrking and was wondering if you guys had any advice to prepare well for a buy-side interview and what would your tips be.

The big problem is that I currently have no stock pitch, and would need to come-up with at least one within 2days and was wondering if you guys had any advice or any ressources you could signpost me to, mainly around coming-up with a thesis.