Didn't he reinstate the Assembly? They should be the ones writing the Constitution, logically.
Morsi ordered the Parliament to be reinstated a few months ago, in defiance of the court order. But they only met for one day, for the purpose of setting up some kind of process for appealing the court's decision. (It's been a while now, and I don't remember all the details.) Then when Tantawi was forced out, Morsi claimed legislative powers for himself, as long as the Parliament was not in session.
Separate from that is the constitutional assembly, tasked with writing the new constitution. I believe they've been meeting on and off for some time now, frequently plagued by both legal complaints and complaints that Islamists hold an effective majority. In his latest orders, Morsi granted them more time to finish writing the constitution.
It looks like there's speculation that Morsi will now use his newly proclaimed powers to reinstate Parliament for good:
http://english.ahram.org.eg/NewsContent/1/64/58946/Egypt/Politics-/New-decree-will-allow-Morsi-to-reinstate-Egypt-par.aspxwhich would be welcome, since otherwise Morsi's powers would be completely unchecked.