WMG / The Tick

The Tick is in the same universe as The Venture Bros. and Dr. Horrible
This has already been suggested by Word of God (one of them, anyway), but to be more precise I think Dr. Horrible is in the same universe as the live-action Tick, while Venture Brothers is the same universe as the animated Tick. This isn't just for practical reasons, it seems to me that the moods and aesthetics of the series' involved just sort of mesh that way.

The Evil Midnight Bomber What Bombs At Midnight is the same person as The Muppet Show's Crazy Harry.
Not only do they look alike and share a love of explosives, but their laughs are also eerily similar.

Obviously, Crazy Harry went even crazier and moved to The City to try to make it as a supervillain.
  • That would explain why he was talking to himself about his shot as a supervillain.

Batmanuel, the mayor of Gotham City, and Richard Alpert are all the same person.
Richard has often left the island on important Others-related business and may have posed as mayor of Gotham for a time in order to obtain new contacts, power, and money. After Batman decides Gotham doesn't need him, Richard dresses up as his own bizarre parody to try and help out before eventually giving up and returning to the island.

The Tick and Codename: Kids Next Door take place in the same universe
No shortage of crazy villains.

The Theme Song was an accident.
The theme song was written as a basic 'the hero is a badass' song about The Tick, but on the day of recording, the singer forgot the words and threw together the first things that came to mind.

The entire show takes place in the mind of a sick child at a hospital
The Creepypasta wiki has an article that explores it more fully, and Cracked.com goes into it in-depth, but the basic idea is that a child was bitten by a tick, got very sick, and now passes the time in a hospital by daydreaming about a superhero and a world built up from whatever is in the hospital room.

The sick child lives in The Venture Bros. Universe.
To expand on the above theories, the sick child at a hospital lives in The Venture Bros. Universe, as it would explain how they can inhabit the same Universe even if one is just a daydream.

The city is a dumping ground for 'useless' superheroes.
But some of them are still competent despite outside appearances, even if it's only in specific areas like Sewer Urchin in...well, sewers.

2016 series

In the new series, Arthur won't be mistaken for a rabbit.
Since his costume is more detailed in the moth motif.

Die Fledermaus and American Maid will appear.