Moon Invasion

Yeah, yeah, yeah... The Moon Lord exists. And? Doesn't mean that the possibility of the moon being a boss is zilch. The Moon Invasion is activated using the Gravitational Decipher Staff which shifts the gravity between the Earth and Moon. The event lasts for over 7 in-game days, but isn't like most other events, as at the beginning of every in-game day a special boss will spawn for you to fight, these are the bosses, in this order:

Day 1 - Wall of Steel
A remake of the Wall of Flesh, is very similar to the Wall of Flesh but has much higher stats and new attacks. The eyes are a bit different now, instead of both shooting lasers, only one does and the eye that does shoot lasers is a red colored one. The other is green and shoots homing balls of magic green fire. The eyes have their own health which both need to be defeated for the mouth in the center to be vincible. Once the red eye is defeated, it turns into the phase 2 version of Retinazer that shoots 3 lasers and is invincible. Once the green eye is defeated, it turns into the phase 2 version of Spazmatism that shoots a big spray of homing green fire and is invincible. The mouth can spawn mini metal worms and the main body has metal hungry mouths that are connected by chains and fly.
Hp: 30,000 (Mouth), 18,000 (Red Eye), 17,000 (Green Eye)
Pow: 200 (Mouth), 190 (Red Eye), 210 (Green Eye)
Def: 70 (Mouth), 60 (Red Eye), 65 (Green Eye)
Speed: Slightly faster than Wall of Flesh 

Message after defeat:
"You feel the air close in around you..."

Day 2 - The Triplets
A remake of the Twins, very similar to them as Retinazer and Spazmatism return with another friend, Seizurism, who is a blue eye. Retinazer B is now upgraded and can shoot 3 lasers at once rather than 1 but nonetheless has the same attacks. Spazmatism B now shoots green fire that homes in on you but has the same attacks. Seizurism however, shoots beams of light that go on forever. In phase 2, Seizurism's retina splits off and a glass prism is shown, now it can shoot not only a beam of light but also 7 colored beams of light that come together into one similar to the last prism.
Hp: 27,000 (Ret.), 28,500 (Spa.), 30,000 (Sei.)
Pow: 240 (Ret.), 280 (Spa.), 300 (Sei.)
Def: 80 (Ret.), 75 (Spa.), 60 (Sei.)
Speed: Fast (Ret.), Extremely Fast (Spa.), Fast (Sei.)

Message after defeat:
"You feel as if all matter is watching you..."

Day 3 - The Eater of Dimensions
A remake of the Destroyer, very similar to it but now the red lights are blue and it now splits into smaller worms every time a segment is defeated. If the worm manages to end up as one segment only then it'll become an enemy similar to Corruptors, Eater of Souls, etc. When a segment is destroyed, a Probe-2 is spawned. Probe-2's shoot 3 lasers at once, rapidly.
Hp: 120,000 (Altogether), 2,400 (Per segment), 2,000 (Probe-2)
Pow: 120 (Per segment), 80 (Probe-2)
Def: 110 (Per segment), 80 (Probe-2)
Speed: Extremely Fast (Per segment), Slow (Probe-2)

Message after defeat:
"The ground beneath you feels like it's shifting..."

Day 4 - Skeletron Omega
A remake of Skeletron and Skeletron Prime, very similar to how the Moon Lord fight works but you're able to fight it's main health immediately. This boss is a giant skeleton in the background made of metal that has two arms, one hand shoots lasers (left), the other shoots fire (right). The head can spawn mini skeletron heads to attack you.
Hp: 50,000 (Head), 10,000 (Left hand), 10,000 (Right hand)
Pow: 100 (Head), 110 (Left hand), 110 (Right hand)
Def: 130 (Head), 60 (Left hand), 80 (Right hand)
Speed: Average (Altogether)

Message after defeat:
"You feel the spirits of time and space wisp around you..."

Day 5 - Planetera
A remake of Plantera, this remake of Plantera is made of steel and metal, and floats in the air. It has 3 "planetary" enemies floating around it, one shoots lasers, another fire and the last causes random debuffs. You need to defeat all 3 planets to fight Planetera. Planetera can shoot thorns at you and use it's chains to hold on to you and sometimes pull you towards it. After phase 1, the blue bulb part of Planetera comes off and 3 new planets start orbiting it. One planet shoots 3 lasers rapidly, the other shoots massive homing fire and the last has light beams. After those are defeated you can fight Planetera once again. In this phase it shoots tiny round meteoroids at you and shoots homing lasers at you.
Hp: 60,000 (Planetera), 12,000 (Planets)
Pow: 130 (Planetera), 75 (Planets)
Def: 120 (Planetera), 100 (Planets)
Speed: Slow (Phase 1), Quick (Phase 2), Fast Orbit (Planets)

Message after defeat:
"Life shudders in fear around you..."

Day 6 - No Boss, only a Warning
On this day, there is no boss that spawns but rather a warning saying:
"The Moon looms closer to Earth..."
Throughout this day, the Moon is always in the sky and slowly coming closer to the planet. At 4:30am on Day 7, the Moon then spawns in the sky as a boss...

Day 7 - The Moon
On this day, the Moon spawns in the sky and begins to slowly come closer to the ground. The Moon would be slower in a small world and quicker in a large world due to the difference in size but nonetheless it's coming down and there are special monsters that spawn. The Moon has no direct attacks other than the damage it does when you touch it. The enemies are distractions. One enemy is the Solarmia which is modeled after the Sun and shoots fire at you, the other is the Lunarmia and is modeled after the Moon and shoots homing blue fire at you, but it's much slower.
Hp: 1,500,000 (Moon), 40,000 (Solarmia), 20,000 (Lunarmia)
Pow: 300 (Moon), 120 (Solarmia), 90 (Lunarmia)
Def: 150 (Moon), 30 (Solarmia), 150 (Lunarmia)
Speed: Extremely Slow (Moon), Fast (Solarmia), Slow (Lunarmia)

Message after defeat:
"The world is at peace... except in the tide-less ocean..."

Rare Event - Eclipsol
There is a rare chance that the Moon can be replaced by Eclipsol which is like the Moon but much more difficult. The grey rock is now pitch black with an orange glow, like a Solar Eclipse. It has the same chance of replacing the Moon as a Solar Eclipse does to spawn in the day. Instead of the Sun and Moon enemies, enemies similar to planets in the Solar System spawn; Earf, Joopita, Satoon and Marsh. Earf sprays blue fire at you, Joopita rams into you, Satoon fires homing rings at you and Marsh shoots lasers at you.
Hp: 2,000,000 (Eclipsol), 60,000 (Earf), 30,000 (Joopita), 25,000 (Satoon), 10,000 (Marsh)
Pow: 500 (Eclipsol), 100 (Earf), 200 (Joopita), 150 (Satoon), 50 (Marsh)
Def: 200 (Eclipsol), 100 (Earf), 75 (Joopita), 100 (Satoon), 175 (Marsh)
Speed: Slow (Eclipsol), Fast (Earf), Average (Marsh), Slow (Joopita), Extremely Slow (Satoon)

Message after defeat:
"The eclipse is no more... and the world is at peace..."

More Variety in Worlds

To make most of these ideas work, I would need the world size to be bigger, otherwise everything would either be small or cramped. Maybe even have a new world size? Small, Medium, Large & Extreme?

Outer Space
  Outer Space isn't very... I don't know, expansive in Terraria? Maybe the world height could be raised so we can actually have an outer space biome with UFO's and aliens as enemies. Maybe even floating bits of rock like small meteoroids orbiting the planet? Just an idea :)

Volcanoes & Mountains
  Maybe we could have real mountains in Terraria that even may be able to be volcanoes? They can be topped with snow and be really tall. The volcanoes however, won't have snow on them and instead have a crater that goes through the mountain and down to the point where lava starts appearing underground with a massive lava pool. These volcanoes could even have their lava slowly raise and if they get high enough, erupt?