This is a list of characters, heroes and villains who have appeared in Astro City stories, but lack some form of firm confirmation of their identity via in-story dialog and captions, or external commentary by the creative staff.
Kurt Busiek's Astro City Volume 1
#2: - On the title page there is a flying individual that is too far away to positively identify. Based on the sharpness of the angle and the year being 1959, it's quite likely Supersonic, but could also be The N-Forcer or Starwoman presuming it's a hero we've seen before.
Kurt Busiek's Astro City Volume 2
#14, #19:- Both issues feature a large group pictures of the Kiefer Square area residents as well as some of the black masks. Some were probably named dropped, like the Muscleman but there were no confirmations.
Astro City: Local Heroes
Astro City: The Dark Age Book One
#3:- It very much appears that the villainous first Quarrel is aiding Simon Magus, Hellhound and the Street Angel in attempting to drive out the vengeance spirit in the Blue Knight. It's only added here due to the figures being drawn small.
#4: - In the major fight scenes during Madame Majestrix's Invasion, there are several characters that are drawn smaller and away from the main fray. One is likely the Unknown Green Guy mentioned above. Another looks to be a giant sized man possibly a size changer. There are also two unknown flying characters. Shrff, Mermaid and Beautie are all possibilities of active flying heroes of the time. Shrff is a real likely candidate as the rest of the Apollo Eleven were there. There is also the possibility of the mysterious fifth member of the Experimentals or even Street Angel leaping around. It's also possible to be two unknown or non-Astro city based heroes as well.
Astro City: The Dark Age Book Three
#1: - On page seven, after the battle between Hellsignor and the two teams of heroes, The First Family and The Omega Rangers, there is a wide shot. The five members of the First Family are easy to pick out, as are Butterfly, Krakaboom, Witchling and Hellhound all of whom appeared in previous pages. But there are two other figures in addition to the Point Man and Cleopatra (Modern). They are likely Quarrel II and Bravo, based on the team membership that we've seen. But there could be additional team members or two other heroes who were captured.
#2:- On page 12, the alternate earth group Olympians is fighting Honor Guard, while Royal Williams lists three of the members, there are at least three members seen that don't get named. One is a male super-speedster dressed in blue. Another looks to be a being of liquid or ice, floating on a platform or board who can project blasts of the liquid or possibly energy. Another appears to be an acrobatic type or someone who can cling to walls. It also appears there could be a team member who can project major blasts, hence why Stormhawk is being knocked backwards.
Astro City: The Dark Age Book Four
Astro City Special: The Silver Agent
#1:- On page three there are a pair of winged costumed flying "birdmen" fighting with Max O'Millions in a scene likely in the late 1940s or the early 1950s.
On page 16 panel 2, there are two individuals fighting with the Silver Agent. Both appear to be green in nature suggesting aliens, though one also appears to have robotic elements in their face.
#2:- On Page 21 panel one, there are two flying unknown/unidentified characters during the return view to battle during Madame Majestrix's Invasion. It's possible they are two of the unknown mentioned above in Astro City: The Dark Age Book One #4.