"Dimension Fusion" cannot Special Summon a "Special Summon-only" monster, like a Ritual Monster or "Chaos Emperor Dragon - Envoy of the End", unless it was properly summoned first.
You cannot activate "Dimension Fusion" when neither player has a monster removed from play; at least 1 player must have at least 1 monster removed from play for you to activate it.
"Dimension Fusion" does not target.
You can use "Dimension Fusion" to Special Summon a monster removed from play with "Different Dimension Gate".
You cannot use "Dimension Fusion" to Special Summon a monster that was removed from play and kept face-down, like with "Lightforce Sword" or "Different Dimension Capsule".
If you take control of your opponent's monster, and it is removed from play while on your side of the field with "Dimensionhole", when "Dimension Fusion" resolves the monster is Special Summoned to the player's side of the field who controlled the monster when it was removed.
You can activate "Bottomless Trap Hole" after monsters are Summoned with "Dimension Fusion"; "Bottomless Trap Hole" destroys the Summoned monsters on your opponent's side of the field, and your Summoned monsters are not destroyed.