As you play through stories in Mission Architect and even as others play your stories, you'll earn Tickets. You can then take these tickets to an Architect Ticket Vendor and redeem them for various rewards. Some of those rewards are unlockable content to use within Mission Architect. Unlocking this content unlocks it for all of your characters. Below is a list of things that can be unlocked with Tickets.
These are full enemy groups such as Axis America, Praetorians and The Winter Horde.
These are packs of unique characters such as The Arachnos Pack that includes a Rank: Boss version of Lord Recluse, Ghost Widow, Mako, Scirocco and others.
These are full map sets, with multiple maps in each set such as 5th Column, Original Caves and Arachnoid Tech.
These are unique map packs that contain similar maps in a group such as the Croatoa Map Pack that contains outdoor maps of many of the Croatoa areas.
These are different costume pieces that you can unlock that allow you to use these pieces on characters within Mission Architect. Examples include, Custom Weapons and Shields, Cimeroran Costume Pieces and Holiday Event Pieces.