The Millie is one of my favorite EDCs as well and if you like it, you could just get another. A brown XHP version would be a cool replacement.
If you want to try something different, I'd recommend:
1. PM2 - The M390 version is still available or a CTS-20CP version is available on the used market
2. Manix 2 in M4 from the same place as the PM2 in M390
3. Endura 4 in ZDP -189. This steel is growing on me and I'm still trying to decide if it or M390 takes a finer edge. The Endura is just a hair shorter than the Millie, but it is noticeable smaller in the pocket.
4. BM AFCK in M390 - There are still blades available in the black coated version. The dimensions of the AFCK and Millie are VERY similar.