The challenge will be, there is no module harder than Empire in Flames because it’s the longest running campaign. Doomstones comes close but it is a very very different campaign.
I would recommend the three adventures Ashes of Middenheim, Spires of Altdorf and Forges of Nuln. Have them take place after the anarchy of the rebellions in Empire in Ruins, rather than the after effect of chaos invasion. The advantage is that several NPCs will already have been referenced.
That’s a fairly substantially mischaracterisation. Every edition had its own set of a large number of adventures. 4th edition is the first time Enemy Within has been reprinted for a different edition and it is 30 years later. Aside from that it’s been a couple of small one off modules like Rough Night at the Three Feathers, Night of Blood or Natassia’s wedding that are reprinted as PDFs or included as a bonus in another book.
New Hardcover/softcover substantive adventures
2nd: Ashes of Middenheim, Spires of Altdorf, Forges of Nuln, Lure of the Liche Priest, Terror in Talabheim
3rd: Witches Song, Gathering Storm, Enemy Within (Totally different story), The Edge of Night
4th: Ubersreik Asventures, Ubersreik Adventures 2.
[Edit] An alternative to adding on to the end, would be got insert some things partway through. Rough Nights and Hard Days is designed to be inserted into Enemy Within and is worth a few levels. Oldenhaller Contract would slide in very nicely to any city or town location and links in later to RNHDs. Pretty much all the Ubersreik Adventures can be easily slotted by in. Hell if you really want to scare your players send them to Castle Drachenfels!