Check out the Ultimate New Player Guide (WIP).

• R Heroes (Rare):
  
  • Eugene
  • Smith
  • Charlie
  • Cloris
• SR Heroes (Epic):
  • Sergey
  • Jessie
  • Patrick
  • Walis Bokan
  • Ling Xue
  • Bahiti
  • Gina
  • Jasser
  • Seo-yoon
• SSR Heroes (Mythic, Generation 1):
  • Jeronimo
  • Natalia
  • Molly
  • Zinman

Polar Terror unlocks

Give or take few days, it depends how fast is your server at completing the required tasks, so it can unlock before the Monument mission runs its course.

• Level 1 - Day 1
• Level 2 - Day 1
• Level 3 - Day 10
• Level 4 - Day 18
• Level 5 - Day 27
• Level 6 - ?
• Level 7 - ?
• Level 8 - Day 150

Arena

Day ~34 (on Monday) - Opponent pool enlarged by nearby servers.

Fortress & Stronghold

Give or take few days, it depends how fast is your server at completing the required tasks, so it can unlock before the Monument mission runs its course.

• Fortress - 4
• Stronghold - Day 24

Another Part of the Map Opens

Give or take few days, it depends how fast is your server at completing the required tasks, so it can unlock before the Monument mission runs its course.

• Tundra - Day 14
• Fertile Lands - Day 39

At day 40, the following aspects of the game unlock/update:

Heroes of Generation 2

• [Infantry] Flint - Lucky Wheel Hero
• [Lancer] Philly
• [Marksman] Alonso

Legendary Hero Gear

New tier of Hero Gear, one above Mythic.

At day 53, the following aspects of the game unlock/update:

Sunfire Castle

Alliances within the state can battle for the control of the Sunfire Castle. When a President is elected, minister positions can be assigned.

• The Sunfire Castle Battle happens on Saturday.

At day 54, the following aspects of the game unlock/update:

Pets of Generation 1

• Cave Hyena
• Artic Wolf
• Musk Ox

At day 60, the following aspects of the game unlock/update:

Fire Crystal Age

When 'Road to Discovery' Monument mission ends, Fire Crystal Age will be unlocked in your state and players at Furnace Lv. 30 can upgrade buildings further.

At day 80*, the following aspects of the game unlock/update:

State of Power (State vs State = SVS)

The main event of Whiteout Survival. Your state will battle with another state for the control of the Sunfire Castle.

• There is also going to be a week long preparation phase which replaces Hall of Chiefs event.
  • SVS Prep Phase starts on Monday.
  • The winning state of preparation phase gets to invade the enemy state and attack their Sunfire Castle.
• One state can hold up to two Sunfire Castles and if that happens, President becomes Supreme President and all the benefits from the Sunfire Castle are enhanced.
• The SVS Sunfire Castle Battle happens on Saturday.

King of Icefield

Event that replaces Hall of Chiefs in the rotation when SVS is on cooldown. Basically the same thing as Hall of Chiefs, but besides the in-state personal leaderboard, there is also neighbouring states leaderboard both on personal and state level.

• King of the Icefield starts on Monday.

*Note: Both events have its own schedule. If your server isn't around the day the events start on at day 80, you will have one more Hall of Chiefs.

At day 90, the following aspects of the game unlock/update:

Pets of Generation 2

• Titan Roc
• Giant Tapir

At day 120, the following aspects of the game unlock/update:

Heroes of Generation 3

• [Infantry] Logan
• [Lancer] Mia - Lucky Wheel Hero
• [Marksman] Greg

VIP Prestige Shop

I actually could not find almost any info online about this shop, but from my understanding, it is a special shop for players who have reached VIP level 12 and it offers various special cosmetic items.

Fortress Battle Rewards Update

Fire Crystals and pet advancement items added to the reward pool.

Vision of Dawn Event Update

Added usage of Golden and Platinum Keys to daily tasks.

At day 140, the following aspects of the game unlock/update:

Pets of Generation 3

• Snow Leopard
• Giant Elk

At day 150, the following aspects of the game unlock/update:

Fire Crystal Age Update

• Fire Crystal Furnace Lv. 4 and 5 and all the corresponding buildings along with it.
• New building - Crystal Laboratory! Allows to refine small amount of resources 5 times a day for random number of Fire Crystals (1-5).

Daily Intel Missions Update

• Increase from an average of 10 Fire Crystals to 20 Fire Crystals a day.

At day 180*, the following aspects of the game unlock/update:

*Most online sources say day 180, but for our server it unlocked at day 170.

Legendary Chief Gear

New tier of Chief Gear, one above Mythic.

Enhancement Material Exchange

Chief Gear Enhancement Material Exchange "Shop" that unlocks if you have at least one piece of Chief Gear at Mythic T2 ***. You can trade Alloy, Solution, Plans and Amber.

More Ways to Obtain Chief Gear Enhancement Materials

• Crazy Joe
  • New, higher difficulties: 12-21
  • Design Plans and Chief Gear Material Chests added to the reward pool
• Mercenary Prestige
  • New, higher difficulties: Legend's Initiation
  • Chief Gear Material Chests added to the reward pool
• Alliance Championship
  • Design Plans added to the Alliance Championship Shop

At day 195, the following aspects of the game unlock/update:

Heroes of Generation 4

• [Infantry] Ahmose
• [Lancer] Reina
• [Marksman] Lynn - Lucky Wheel Hero

Fortress Battle Rewards Update

Molly and Zinman shards replaced by Philly and Flint shards.

Daily Intel Missions Update

Intel missions award mythic shards of heroes from generation 2 lower than the current generation. For example, at Hero Generation 4, the mythic shard rewards from Intel missions will be for heroes from Generation 2.

Hero Hall Recruitment Update

Molly shards from chests are replaced by shards for mythic heroes from generation 2 lower than the current generation. For example, at Hero Generation 4, Molly shards are replaced by shards for heroes from Generation 2.

Event 'Gina's Revenge' Replaced by 'Hero's Mission'

Similar event, but instead getting Gina shards, you get shards for a mythic hero from the generation 2 lower than the current generation.

• You can now exchange Horn of Cryptid for Hunter Pouch (was only possible before if you had Gina maxed out).
• You can now exchange Horn of Cryptid for 2 Gina Shards.
• You can now exchange Scattered parts for 2 Gina Shards.
• You can now exchange Scattered parts for Loot (same rewards as Hunter Pouch).

At day 200*, the following aspects of the game unlock/update:

Pets of Generation 4

• Cave Lion
• Snow Ape

Alliance Shop

• The weekly offer of 1K Pet Food increases from 5 available items to 10.
• The weekly offer of Pet Advancement Materials Custom Chest increases from 4 available to 7. 

*In some states, these pets could unlock at the same time as Heroes Generation 4 arrives.

At day 220, the following aspects of the game unlock/update:

Fire Crystal Age Updates

• New Building - War Academy!
• Crystal Laboratory Lv. 5 - Up to 6 Fire Crystal refinements per day (from 5).

At day 270, the following aspects of the game unlock/update:

Heroes of Generation 5

• [Infantry] Hector - Lucky Wheel Hero
• [Lancer] Norah
• [Marksman] Gwen

At day 280, the following aspects of the game unlock/update:

Pets of Generation 5:

• Iron Rhino
• Saber-tooth Tiger

At day 330, the following aspects of the game unlock/update:

Fire Crystal Age Updates

• Introduction of Refined Fire Crystals needed for Fire Crystal upgrades of levels 6 and higher.
• Fire Crystal Furnace Lv. 6, 7 and 8 and all the corresponding buildings along with it.
• Crystal Laboratory Lv. 6 - Up to 7 Fire Crystal refinements per day (from 6).

At day 360, the following aspects of the game unlock/update:

Heroes of Generation 6

• [Infantry] Wu Ming
• [Lancer] Renee - Lucky Wheel hero
• [Marksman] Wayne

At day 440, the following aspects of the game unlock/update:

Heroes of Generation 7

• [Infantry] Edith
• [Lancer] Gordon
• [Marksman] Bradley - Lucky Wheel hero

Chief Charm

Unlocks Chief Charm Level 16.

Unlocks Chief Charm Exchange Shop - need to have at least one Chief Charm at Level 11.

At day 500, the following aspects of the game unlock/update:

Fire Crystal Age Updates

• Fire Crystal Furnace Lv. 9 and 10 and all the corresponding buildings along with it.

At day 520, the following aspects of the game unlock/update:

Heroes of Generation 8

• [Infantry] Gatot - Lucky Wheel hero
• [Lancer] Sonya
• [Marksman] Hendrik

At day 600, the following aspects of the game unlock/update:

Heroes of Generation 9

• [Infantry] Magnus
• [Lancer] Xura
• [Marksman] Fred

Unlock Legendary T4 Gear.

More ways to obtain Chief Gear Enhancement Materials:

• Crazy Joe - more rewards.
• Mercenary Prestige - new Legendary difficulty unlocked.
• Alliance Championship - more items amount in the shop.