I thought our Defensive Line play was good this year. We were fairly stout against the run and were league leaders in sacks, that doesn't happen without solid play on the line. I thought our biggest liability on D was in giving up the big passing play and huge games to TE's. That to me says we need more speed and coverage ability at LB and improved safety play.
At LB Miller eats up one spot and I expect him to improve in coverage again next year as he did this year, but do you really want him in coverage over rushing the passer? Answer is NO. WW takes another spot and he is about as fast as it gets at LB so I don't see a big change there next year. Where we lack speed and ability is at MLB so that is the obvious upgrade.
At Safety we again have a few decent parts, but no game changers (either from a fear of getting clobbered ie.. Pollard type player or a ball hawking player).
Those are the two biggest areas of need on the Defense, not DT IMO.