While I understand what you're saying, there have been numerous suggestions (even recently) about this.
The issue really is balance. Even one slot extra would completely throw a wrench in the "level 50 SO-slotted" game. That's how it works, even with the presence of IOs things are still reasonable for most folks.
Adding slots, no. That would give too much imbalance potential.
Kheldians, I think though, should start out with slightly different slotting potential, overall. I do hate not having reasonable slots on everything, having to sacrifice one for another form. But - again - they're balanced to that level 50 certain number of SO's slotted.
So while I kind of like the idea, it's not practical and would seriously change the game for those who are willing to grind and get all kinds of drops/inf, versus those who don't - and IO's are meant to be optional, not required.
Please read my FEAR/Portal/HalfLife Fan Fiction!
Repurposed
Had an idea going through my head past couple of days, and don't remember seeing it anywhere before. Someone on a team mentioned needing the level cap raised. I think a lot of people think raising the level cap would make them more powerfull, personally I don't care anything about grinding another 10 levels or so. Now slots on the other hand, I think everyone could use more of. What if TF's and trials went back to a reward window when you finished. You could choose from a random IO roll, merits, or slots. When you picked slots, you would be rewarded 3-5 slots, and you would go to a trainer and claim them, placing them would be like leveling up. Just think of all the places we could put more IO set bonuses, and I know my WS and PB would love it. What do you think?