Speaking mostly for myself here, some characters we would love see:

MARVEL COMICS / CINEMATIC UNIVERSE
CURRENT: Sean Cassidy (616), Kaine (616), Peter Parker (Ultimate), Steve Rogers (MCU), Natasha Romanoff (MCU), Sif (MCU)
WANTED: Bruce Banner, James "Bucky" Barnes, Peggy Carter, Nick Fury, Maria Hill, Loki, Pepper Potts, James "Rhodey" Rhodes, Thor, Howard Stark, Tony Stark, Johnny Storm, Sam Wilson

BUFFYVERSE
CURRENT: Winifred Burkle, Tara Maclay, Buffy Summers, Spike
WANTED: Angel, Drusilla, Rupert Giles, Charles Gunn, Faith Lehane, Willow Rosenberg, Wesley Wyndam-Pryce, Dawn Summers, Joyce Summers