Grinding Gear Games is aiming to put out a mini-expansion every 4 months between their 1-2 week update cycles of new skills, fixes, content, and more. The details for the first expansion which will go live March 5th, have been leaked.

Summary of the new Expansion:

• Vaal orbs can corrupt gear (modify gear - adds an intrinsic line? corrupted boots earlier had intrinsic MS%)
• Vaal orbs can corrupt gems (makes them uber)
• Uber gems require souls to cast (roughly 14 new skills)
• Vaal artifacts occur every so often during gameplay and corrupt the area around them
• Corrupted areas will have new "bosses" (?) and is where you will find vaal orbs

• AMBUSH league (softcore): treasure chests guarded by monsters - monsters can be customised? (like maps but for chests)
• INVASION league (hardcore): new challenging bosses with new mechanics appear!
• Vaal artifact fragments: complete the artifact to unlock new area(s)
• APEX OF SACRIFICE (a new dungeon?): first 100 to kill the boss in here get a cosmetic reward!
• PVP MODES!: cutthroat mode (unsure if it is its own "league") and FFA arena