There're a lot of OOBs currently, so many, than unless you spent a good amount of the budget on them, you will take a long time to research them, however, in my latest game I found than the game is critically lacking some very important OOBs.
* Motorized AT guns: since we can have motorized arty, why not AT guns? they need to move a lot faster since they will be defending the frontline from tanks, which normally are faster than infantry.
* Mechanized Assault brigades should let you select to use mechanized arty instead of regular arty, since the whole point of the OOB is having hard hitting units.
* There should be an OOB of "armored assault brigade" which could have light/medium/heavy tanks along with mechanized arty.
* There should be an OOB of Mgs with either arty or AT guns, which could be on foot, motorized or mechanized with APCs.
The one than its more urgently needed its the Motorized AT guns, the other ones are just suggestions.