They based the characters’ on storyline, not the overall plot of the DOA series. Next to storyline they also picked characters based on popularity. Leon just wasn’t standing strong in both of those categories, same with Gen fu and Alpha-152. Momiji and Rachel were strong in the popularity category but since they’re considered guest characters, they weren’t added. We can all cry about Leon, but he honestly wasn’t memorable. My personal DOA6 roster would be:
+ 2 new DLC entries (Jin Kazama + Kiryu Kazuma).
I personally would’ve rewritten a few characters.
I would make Marie a more serious character, more focused on working, doing research for Helena as her right hand instead of her lap dog.
Nyotengu woud be focused on trying to kill Hayabusa for killing the other Tengu, but would fight Momiji and Rachel instead.
Multiple Phase 4 clones would have a showdown with Hayabusa.
Raidou would’ve had a showdown with Ayane and Hayate.
Kasumi would’ve been focused on taking down NiCO, Donovan, Rig and Christie with Bayman, Helena and Marie Rose.
The rest would’ve been like the story regulars, minding their own business and doing their stuff.