Hard to say, really. DFG is 2600 gold...you could go with Abyssal Scepter, for 2650, but getting in range for the aura to proc isn't always easy. (Helps against people who jump you, though.) Switching to Morello's Evil Tome will impact your build the least, in terms of keeping the AP, CDR, initial Kage's, and comparable price (2330 gold,) but I don't know if that provides the oomph you're looking for. You could replace it with Void Staff (2295) for damage or Will of the Ancients (2500) for spellvamp, if those aren't already part of your build, or maybe you could just rush them faster if they are.
If you're looking for something to replace the offensive power of DFG, I'd go for Abyssal or Void. If you're looking for something with extra survivability so you can survive longer to make up for your lack of damage, I'd go for Abyssal (situationally) or WotA. If you're looking for the CDR, then Morello's is probably your best choice. If you're asking me for my personal opinion, I'd try replacing it with an earlier Void or with WotA.
Take heart, though. Morello's plan was to remove DFG and buff burst casters so they don't need it anymore. DFG has not been removed, but it HAS been nerfed, so chances are good that burst casters are going to get buffed at some point in the not-too-distant future to make up for it.