So the options im considering are
1. Aegis defense line with the devastators and adding another marine.
2. A Librarian in terminator Armour
3. Another predator or vindicator
I'd go for the Aegis line, which then gives you leftover points to spend on bulking out either your bike squad or your Devastators. The Librarian would need to go with the Terminators to be of any real use, and then you'd have an eggs-in-one-basket unit.
A tri-las Predator would also be acceptable, as you're a little lacking in long-ranged anti-armour firepower, but it's better to have flyer-insurance, and the quad-gun can always track ground targets. If you stick your Scouts behind the Aegis line, it'll free your Devastators up to target other things while the Scout Sergeant mans the gun.
Have flak misiles been introduced yet for anti flyer?
At the moment, only Dark Angels and Chaos Space Marines have access to flakk missiles.