Once Isaiah Thomas was inserted into the starting lineup in the fourth game of the 2015-16 season, the Celtics became his team and were propelled to the fourth seed by the guard’s first all-star campaign.

Thomas’ tremendous season — ending with career-highs in starts with 79 games and games played at 82 — provided a full display of his ability to shoulder the offensive load for a playoff team. Nylon Calculus’ True Usage rating, an estimate of how often a player contributes to the result of an offensive possession, ranked him 10th in the league, just slightly below LeBron James and Steph Curry.

Photo courtesy of nba.com

Photo courtesy of nba.com

Complementing Thomas has been Jae Crowder, the main win from the Rajon Rondo trade who solidified himself as a starting wing who is capable of playing consistent on-ball defense and showcased his defensive awareness for opponent passes with 1.73 steals per game this season.

While Crowder’s shot wavered in the playoffs, he hit just over 33 percent on three pointers during the regular season on five attempts per game, both career highs. While possibly aberrational, Crowder displayed decent form and would benefit from a demotion in the offensive rotation.

Avery Bradley showed a vastly improved offensive game, scoring a career high 15.2 points per contest with an effective field goal percentage of 52 percent, the best efficiency since his sophomore season.

Bradley continued the Brad Stevens-induced trend of substituting three point jumpers for what had been long-twos, contributing to his uptick in scoring efficiency. The former Longhorn put forth another superior defensive season, which was highlighted in the Celtics’ two clashes with Golden State in which he starred in his audition as the league’s best Steph Curry defender.

The development of Thomas, Crowder and Bradley asserted them as the above-average NBA starters. Paired with a rim-protector and another shot-creator, Boston has the beginning of an elite starting lineup. The flaws of one dimensional players such as Marcus Smart and Kelly Olynyk don’t look so glaring when they are relegated to bench roles.

With free agency rolling around again this summer, the true question is: What additions will the Celtics make?

The free agency market is ripe with players to fill the Celtics’ needs and the suitors will be beyond plentiful. Where Danny Ainge directs his attention in the early days of July will be fascinating. Without further ado, here are a few suggestions as to who Boston could possibly pursue:

Potential Position: Center

Flushed with cap space, the Celtics can afford to target the defensive presence at the rim they’ve sorely missed since the departure of Kevin Garnett. While the Raptors’ center has played himself into a max contract in the series against the Cavaliers, he will be worthy of an overpay to take a good defense built on effort and intelligence to an elite level.

If the Celtics target the block machine, however, they will need to prioritize scoring and ball-handling on the wing as they will receive a little to no contribution from Bismack Biyombo on that end. Biyombo’s hands are bad, but the staff could help him develop a slightly better lob catch, turning him into a decent rim-runner.

Though there is uncertainty concerning Biyombo’s age (23), he’s shown nothing to suggest his game will suffer a large drop off over the next four years. A team nestled up against the cap with one big move left would be remiss in putting all their eggs in Biyombo’s basket; a team like the Celtics, though, with several rookie scale and bargain veteran contracts extending through the cap boom, should feel comfortable targeting him.

Ian Mahinmi is another option for the C’s, possibly at a slightly lower price tag as he is Biyombo’s senior by a decent margin at the age of 29, though he offers less mobility on the perimeter and functions more as a point-blank rim protector.

Neither Hassan Whiteside nor Dwight Howard seem to fit the Celtics’ faster style of play, and too much uncertainty surrounds Festus Ezeli’s health for Ainge to lure him away from Golden State with a substantial offer. Joakim Noah would be a good fit on the floor, adding the best playmaking and defense Brad Stevens has had at the center spot during his coaching tenure.

Timofey Mozgov and Donatas Motiejunas are possible reclamation projects, too. Mozgov could still flourish as a force in the lane with four shooters around him, and Motiejunas would allow the Celtics to go five-out for the entire game with Olynyk as his backup. Both players might be available on cheaper contracts after disappointing seasons, but with so much cap space around the league they could be bound for expensive deals that aren’t cost-effective for Boston.

Miles Plumlee is also a viable option and would be a good target if the Celtics find this crop of centers unappealing compared to 2017 or 2018. The latter class has a great deal of star power at the five, including Derrick Favors, DeMarcus Cousins and Brook Lopez.

Potential Position: Wing

The Celtics will make a pitch for Kevin Durant, but they can’t realistically offer him a better opportunity to win than the Thunder, making plans B, C and D very important.

Jared Dudley should be a priority for Boston; they can spend heavily on a one or two year deal or get a bargain on a longer contract. Dudley, turning 31 this summer, should have four years of consistent basketball left in him, though injury concerns make a hefty deal doubtful.

The Boston College alum would be best served coming off the bench as he is able to replace Crowder and also play alongside him in a smaller unit that could easily switch on defense, especially if paired with Marcus Smart at the shooting guard.

Marvin Williams is appealing for similar reasons, as the four man offers a reliable three point shot and switching ability as a player who has been a small forward at times in his career. In a contract season, Williams was a top 10 power forward in ESPN’s real plus-minus rating, but whether or not that’s replicable is certainly up in the air. He is passing the age 30 threshold in 2016, making a long team deal his priority. However, he is a questionable proposition for a Celtics team looking to maintain future flexibility.

From the restricted market, Solomon Hill would be a nice addition as a versatile defender, though the Celtics could use someone with more offensive gravity. Moe Harkless is another option, though the Blazers will be likely to match any offer within reason.

The most impactful possibilities on the wing are Chandler Parsons, who will exercise his player option and become a free agent, and Nicolas Batum. The Mavericks’ small forward would bring juice to the Celtics offense as his silky shot and ball-handling ability would fit seamlessly with Stevens’ five-out aspirations.

Batum would fit for similar reasons, though he seems committed to staying in Charlotte and carrying the offensive burden he received in his first year as a Hornet, the most substantial of his career.

Both will hopefully hear from Ainge, as players of their ilk are to be coveted by every NBA team in 2016. Though there are no star players in this class apart from Durant, adding Parsons or Batum would still vault the Celtics meaningfully higher in the East hierarchy.

With guards a complete non-priority for Boston, they will be free to focus their spending on the frontcourt while hopefully maintaining enough cap space to bring back Amir Johnson and Jonas Jerebko. Both have guarantee dates of July 3, two days after the free agency moratorium begins, giving the Celtics time to strike before deciding their fate.

Adding Dudley and Biyombo on one- or two-year expensive contracts — allowing them to return to free agency while there’s an obscene amount of cap space available — would be a sound strategy for both sides. If long-term deals are necessary, both should be valuable enough to prove as trade assets during their first years in Boston, allowing the Celtics to welcome any star player made available for trade with open arms.