A system I would have prefered to have seen implemented:

1) Keep the current 9 slots / 12 slotcost system.

2) Slotcosts don't increase for types of items.
For example: a level 1 ring will have a slotcost of 3. A level 50 ring will still have a slotcost of 3.

3) A better increase of stats of higher level item. You will always want a combination of higher level items over a combination of lower level items!

4) HQ Items: slotcost -1.
Combined with the previous ring example: a normal ring would have slotcost 3. A HQ ring would have slotcost 2.