My guess wold be 4-2-5. Our secondary is just too good not to have those guys on the field, plus the linebackers (other than Vaughn) are very young.
LBs will be young but I also think we will be more athletic. In general, regardless of the formation or lineup, I think Heacock is aiming to get more pressure on opposing QBs this year. If there is a knock on his system it's that it (often) gives QBs too much time to throw the ball. That makes it harder for DBs to cover (even with good coverage) as it gives receivers time to find soft spots or break a defender down, but it also makes it harder to create TOs from rushed throws.
I have a feeling if you asked Heacock in confidence, and he answered honestly, he'd admit he MAYBE missed an opportunity to create matchups that would allow his best players (ends and secondary) to make more plays and create more TOs. That's a pretty fair criticism.