The things I would love to see in DOA6, and this is something I hope I can get My feedback inserted for the game especially for future DLC support.
Classic Costumes: What I would prefer to see whether they're defaults, unlockables, customization, or DLC is the whole "Everything is Back" where they include every single costume design within Team Ninja & Koei Tecmo past from DOA1-DOA5LR (plus some Xtreme swimsuits). From costumes we expect from previous titles that made multiple appearances to ones we ever saw again.
Costume Themes: This is something I tend to expand from New Costumes discussion, but in short is to expand certain costume themes that mainly just had one/same outfit design (Gym, Cheerleader, Bunny Swimsuits, KOF etc.) and give them similar treatment to certain Swimsuits, Contests, & Third-Party Costumes which each outfit is different whether their full clothed or skimpy designs.
Counter Command Training: This would be something where players can no longer having to hold panic or tries to guest what to hold, Counter Command Training where you're fighting against characters attack animation where you go through trails to hold certain attacks from basic (like command training), chain attacks, and combos.
Story Mode: I would want for them to do a mix from DOAD & DOA5 for Major Story Mode for the Main Characters (like Ninjas & DOATec vs MIST), and Character Story for Side Characters (like Jann Lee, Tina, Nyotengu etc.) & MIST Characters (Christie, Rig, Phase-4, & Raidou).
Extra Music Tracks: I still would want them to continue from past DOA music, I would love to see some that wasn't in DOA5 from Non-Returning Characters (like Tengu, Genra, Kasumi a, & Shiden), DOA++, DOA Remixs from DOAU (Ray House) & DOA4 (Crash Club).
Stages: Would love to see New Stages that are like previous title where they were like maps, along with some unique hazards, and hills/stairs (DOAU-DOAD).