Universe 1
-What happened to U1 Kami and U1 Piccolo? What happened to U1 Old Kai? If U1 Old Kai is still alive, why did the other U1 Kaioshins no tried to break the U1 Z Sword?
Universe 2
-Is U2 Lavos (or U2 Time Devourer) going to make an appearance on a DBM U2 special or a DBM minicomic?
Universe 6
-What happened to U6 Lord Slug, U6 Hoi, U6 Hiduregarn, U6 Broly, U6 Babidi, U6 Dabura and U6 Buu?
Universe 8
-What happened to U8 Lord Slug, U8 Hoi, U8 Hiduregarn, U8 Bojack, U8 Broly, U8 Babidi, U8 Dabura and U8 Buu?
Universe 9
-What happened to U9 Cell, U9 Lord Slug, U9 Hoi, U9 Hiduregarn, U9 Bojack, U9 Broly, U9 Babidi, U9 Dabura and U9 Buu? I wonder what happened to U9 Cell.
Universe 10
-What happened to U10 Kami and U10 Piccolo? What happened to U10 Old Kai? If U10 Old Kai is still alive, why did the other U10 Kaioshins no tried to break the U10 Z Sword?
Universe 11
-What happened to U11 Lord Slug, U11 Hoi, U11 Hiduregarn, U11 Bojack and U11 Broly? Did U11 Babidi tried to majinize U11 Lord Slug, U11 Hoi, U11 Hiduregarn, U11 Bojack and U11 Broly?
Universe 12
-What happened to U12 Lord Slug, U12 Hoi, U12 Hiduregarn, U12 Bojack and U12 Broly? Is U12 Buu still alive?
Universe 13
-What happened to U13 Lord Slug, U13 Hoi, U13 Hiduregarn, U13 Bojack, U13 Broly, U13 Babidi, U13 Dabura and U13 Buu?
Universe 14
-What happened to U14 Lord Slug, U14 Hoi, U14 Hiduregarn, U14 Broly, U14 Android 16 and U14 Android 19?
Universe 15
-What happened to U15 Lord Slug, U15 Hoi, U15 Hiduregarn, U15 Bojack, U15 Broly, U15 Babidi, U15 Dabura and U15 Buu?
Universe 16
-What happened to U16 Lord Slug, U16 Hoi and U16 Hiduregarn?
Universe 17
-Did any of the U17 Cell Jrs. ever growth to adulthood? Can the U17 Cell Jrs. actually growth to adulthood? What happened to U17 Lord Slug, U17 Bojack, U17 Babidi, U17 Dabura and U17 Buu?
Universe 18
-What happened to U18 Cell? U18 Krillin and U12 Trunks never directly attacked U18 Cell's body nor did they have (at the time) the powerlevel required to destroy U18 Cell's core. Sure, they destroyed Gero's laboratory with ki blasts when they were outside that laboratory but their ki blasts did not directly target U18 Cell's body.
Unlike U12 Cell, U18 Cell's body was never show to be directly targeted by a ki blast.
Universe 19
-What happened to U19 Lord Slug, U19 Bojack and U19 Buu? Zen Buu showed that Heliotes's armors energy are compatible with Buu's body, so did U19 Babidi tried to use U19 Bojack's gang energy and those Heliotes's armors energy to try to unseal U19 Buu?
Universe 20
-Who froze the U20 Broly who survived a black hole?
U8 i don't know gero just gives up
Gero's motivation is avenging his own son.
Even if Goku dies, Gero still has Master Roshi and Ox King to blame for killing most members of the Red Ribbon Army, which is why U9 Gero still made the Androids despite U9 Goku never arriving on Earth.
I believe Salagir answered Babidi just didn’t find people with enough energy to revive Buu in U6 and U8.
Universe 6 has U6 Bojack, U6 Zangya, U6 Bidou, U6 Bujin, U6 Shin, U6 Kibito and U6 Hirudegarn to give enough energy to revive U6 Buu.
It's very possible the other androids never awaken, Cell included.
Universe 9 showed that even if Goku is dead, Gero would still make the androids anyway. I wonder what happened to U9 Cell.
Or maybe Freeza blasts the Earth to terrorize the surviving Z Fighters.
IIRC the only surviving U8 Z Fighter on Universe 8 is U8 Master Roshi, because U8 Goku was killed by U8 Frieza, the other U8 Z fighters were killed by the U8 Ginyu Force. I think U8 Frieza has no reason to consider U8 Earth a treat, because all U8 Earth survivors are weaker than U8 Cyborg Tao.