I think Moriarty is definitely dead. I just don't see how he could have faked that...
Sherlock would have tried to think of every outcome possible for what he knew would be his last confrontation with Moriaty (I think he knew for a while that it would end in his death - whether real or fake) and knew that there was a possibility that Moriarty wouldn't survive either. There has been a few theories about him texting someone before he faked his death --- what if he was informing Molly that Moriarty's body was on the rooftop? After all, wouldn't Molly need a fake dead Sherlock? There was a lot of eye-witnesses that saw Sherlock 'die' so it's not like people would be lining up to identify the body. Maybe only Mycroft for that matter, and it's not like he would rat out his own brother (at least not again, if he even did the first time). People could assume Rich Brook simply went into hiding, or a witness protection program, and wanted to avoid the public after being traumatized by Sherlock.
Or maybe it's a much more simple explanation --- there was no reason for them to check the roof yet. Sherlock was recently proven to be a so-called-fraud, so his suicide wasn't exactly surprising. Moriarty's body just might not have been found yet.
If there is one thing I'm 99% sure of, it's that Moriarty is dead. The Reichenback Fall may be the last time we ever see Moriarty, but it'll be interesting to see how his actions have impacted the next series and Sherlock himself. Though I did like Moriarty, I'm a little glad he's gone. He wasn't just burning Sherlock's heart, but my own too
...oh, and hi, I'm new here