1. Tessai Tsukabishi - he uses in the first arc some bad ass kido, we never get to see his zanpakuto and seems the manga will end without him.
2. Mashiro Kuna - no info on her zanpakuto and no development on why she was able to maintain hollowfication so long. Also, apparently, a missed chance for a love story with Kensei.
3. Kido corps captain and vice-captain - I highly doubt that the current state of affairs allows them to stay hidden and not to take part in the fight.
4. Kūkaku Shiba - a supporting character, head of the former fifth Noble House of Soul Society. A missing arm, no background story and no demonstration of her real power, except skillfulness.
5. Orihime Inoue - can basically negate anything, i.e. special powers, wounds, destruction and even death itself. She was considered uber-important by Aizen and apparently by Yama-ji himself, enough to split their forces in the final fight. Now she's just cleavage.
6. Vasto Lorde - we know only one of them for sure, who is now an Arrancar, Harribel. Baraggan Louisenbairn was apparently a self-proclaimed king and his number in the Espada suggests he wasn't the most powerful Hollow out there. Also no evidence at all that he was a Vasto Lorde.
7. Shūhei Hisagi - he apparently has developed Bankai and the 69 tattoos duel with Kensei would of been awesome.
8. Izuru Kira - a quiet man who kept to himself. Most of the Seiritei didn't know his shikai ability. He could have developed Bankai and it would have been magnificent.
I leave it up to you to continue the list, guys!