As some people have asked I will make a sub-guide on game-breaking items. What to avoid and what we know works :
Long story short, you can add 3 sockets to Weapons, Off-Hands and Helm and it will never fail.
You can create stronger [NORMAL QUALITY] weapons that are significantly more powerful than Legendaries in some cases; think level 70 item with 12 attributes instead of 4 major and 2 minor.
YES YOU CAN stack multiple matching attributes with almost any item. Not sure where the limits are with each item type yet.
BE CAREFUL WHEN ADDING ATTRIBUTES NOT FOR A SPECIFIC ITEM SLOT, it will indicate errors and the game will act erratic.
YOU CANNOT SOCKET A LEGENDARY GEM, GEM IT AND THEN PLACE INTO AN ITEM SOCKET. You can place a gem inside a socketed legendary gem, it will freeze the game when trying to insert into gear.
How do I create a Gem of Ease that makes me godly?
1. Add Gem of Ease by ID with 12+ attributes
2. Modify Attributes, add x - x Damage (x3), add 150%-220% Critical Hit Damage (x3), add 13% Critical hit chance (x3), add Reduced Cooldowns (x3), you get the idea
----Please note, this is not an exact science. In most cases, Legendary Gems can take almost all affixes but I am not certain to what degree. I will be testing this sson.
3. Enter game and place in socket.
What if I want to go further?
Make any weapon with 12 attributes and 3 sockets, stack the target stats of your build. socket Gem of Ease and 2 flawless royal emeralds
Still not enough?
Create a gem of "insert gem name required for build". Yes that is right, in total you can have 4 godly legendary gems. Don't forget the 3 sockets total in weapon, 3 sockets total in off-hand, 3 sockets in helm. I assume that sockets can be added to more gear, and most item slots will allow 3 slots. I have no idea but that is a lot of main stat on some pieces.
But guy, that still is not enough...
Working on it..