In the mothergame, there is no limit to the number of Ioun Stones one may have active at one time. The only thing is that like bonuses don't stack, so there is no point to having two (2) Deep Red Ioun Stones active at the same time.
In DDO, the Ioun Stone occupies the nonsensical Trinket slot, limiting a character to one active Ioun Stone at a time. This is not what the case should be.
I'd like to propose that you increase the number of Ioun Stones that a character can use. To stay in the spirit of pointlessly restricting items in DDO, let's say one (1) Ioun Stone every two (2) levels. So a level 20 character would be able to simultaneously utilize 10 Ioun Stones. Duplicate stones would not apply any benefit (because I know some Dev would freak out over the idea of a character with 5 active Pale Lavender Ioun Stones).
The Ioun Stone would STOP occupying the trinket slot and would instead have it's own dedicated Inventory tab. I don't have the shop skills to do a mock up, but you'd click on the tab, and it would pop up a list of slots, kinda like how bags work. You'd drop the Ioun Stones in those slots.
You could even make it so you could purchase open slots in the Turbine Store for a piddly amount of TP for those who don't want to wait to gain the necessary levels.
Considering the insane levels of power creep in the game and the gear, including rangen loot, that is available, this would not overpower characters and would be moving closer to the mothergame, which should always be the goal.