Holy shit, I forgot about Team battle lmao. I felt Team Battle was very underused too actually. Mode could essentially be it's own 3D KOF, you could pick 3 characters without the need of a tag function like it's a Marvel game. You are able to play multiple characters in the same match while still following 1v1 official rules. Downsides to team battle was that it was stage limited (tag sorta had that problem for balance reasons I hear) and had the option for waaaay too many characters to be used in a match. I'm in the middle of it returning or not at all, but it was heavily underused because you couldn't go online with it and people forget it's there which TN is to blame for it on that one lol. Matches might be pretty long too (?). I remember playing with Ichiron and Cygnus at Next Level with it and the matches took quite a while from how we felt...but we also played with 8 characters slots so there's that lel but generally DOA can go pretty fast randomly as well. Middle stuff.
If DOA6 had it, I'd imagine it can be used for like 3 characters to share the same meter, one character is the anchor like how KOF does it or one character builds the meter for the next character to make full use of it. Very underrated game mode. It's still pretty untapped stuff for DOA honestly. Was DOA also the first one to do it in the 3D format? (??? don't remember if Tekken ever did it??). The mode is already there, it's just thrown in there lol.
Edit: DOA2 and DOAD had it.
What KL said. I didn't care for it as a whole, but I definitely felt it was heavily underused and untapped though.