Quarterback - With Matthew Stafford, Dan Orlovsky and Kellen Moore all back, this position appears to be set. Perhaps the Lions will sign an undrafted quarterback as an extra camp body, but I can't imagine they will draft a QB this year. There was some speculation that they might draft a developmental QB last year, but that didn't end up happening, and they seem comfortable with their current trio of signal callers.
Running back - With only Joique Bell, Theo Riddick and George Winn, the Lions need to come out of the draft with a running back. Ideally, they would target a back on the second day of the draft. Of course, we can't rule out the possibility of someone in the first round if Todd Gurley were to fall to the 23rd overall pick, and we also can't rule out the possibility of the Lions not adding any running backs on the first two days of the draft. It's a safe bet that they will draft one at some point, though.
Fullback - Emil Igwenagu is the only fullback currently on the roster, so it honestly wouldn't be shocking for the Lions to use one of their final few draft picks to address this position. At the very least, they will likely sign an undrafted fullback as added competition for training camp.
Wide receiver - The Lions have hosted several wide receivers for pre-draft visits this year, indicating that they have interest in taking one in the draft. It's worth noting that most of those receivers are also return specialists, so that may be the main factor if they decide to draft one. In any case, it seems very possible that the Lions will add a receiver at some point. I wouldn't expect them to draft one before Day 2, and if they are mainly focused on the return aspect, they will likely just wait until Day 3 unless they can get someone like Tyler Lockett in the second or third round. I suppose that line of thinking should be thrown out the window if a top receiver like DeVante Parker falls to the 23rd overall pick, though. Then the Lions could upgrade the No. 3 spot and potentially add a future replacement for Calvin Johnson.
Tight end - Like quarterback, this position appears to be set. The Lions already have Brandon Pettigrew, Eric Ebron and Joseph Fauria, so there's no real need at tight end. I suppose the Lions could always draft a tight end/fullback/H-back hybrid type of player with one of their final picks, but it wouldn't exactly make a ton of sense to add a true tight end unless they are planning on moving on from Pettigrew sooner rather than later.
Offensive tackle - With Riley Reiff's future at left tackle and LaAdrian Waddle's health uncertain, this is one of the Lions' top needs. The Lions have several intriguing options already on the roster at this position, but there's a need for more talent. As a result, offensive tackle seems like a definite possibility for the Lions' first-round pick. I suppose it will come down to what their big board at tackle looks like and who is actually available at No. 23, but the Lions seem to have interest in upgrading this position early in the draft.
Offensive guard - Considering the Lions don't currently have a single true left guard on their roster, it's safe to describe this position as a top need. Larry Warford is locked in as the starting right guard, but they have nobody on the left side and no depth to speak of overall. With this in mind, I would expect the Lions to add an offensive guard at some point, and Day 2 of the draft in particular seems like an ideal time for them to address this position considering they selected both Warford and Travis Swanson in the third round the last two years. (Regardless of what happens in the draft, I would expect the Lions to sign a veteran guard like Rob Sims at some point before training camp.)
Center - Speaking of Swanson, he is currently penciled in as the Lions' starting center. Perhaps he could move to left guard if the Lions end up drafting Cameron Erving in the first round, but that's really the only scenario where that seems like a possibility. Outside of that, Swanson should be the starting center, and if the Lions were to take a center at some point later in the draft, that player would likely only be backup depth for the interior O-line.