I think a lot of it will come internally, like retaining Fournier and integrating him more fully into the team, and continued growth from some of our youngsters. Signing Fournier to a 68/4 kind of deal would be ideal for me.
I'd also like to see us make a trade like so:
https://tradenba.com/trades/g1E15aPYyBoston trades: Grant Williams, Tristan Thompson, Carsen Edwards, 2022 top-5 protected 1st
Memphis trades: Jonas Valanciunas
Memphis get a first round pick and some cheap role players, the Celtics get JV. JV isn't really in the same age window as the rest of Memphis' roster, but he's averaging 17/12.5 in 28MPG. My dream would be for him and Rob Williams to split the 5 rotation down the middle, 24MPG each, or maybe 26/22 towards JV. He'd take a lot of the physical pressure off of Rob with his strength and brawn, and he's a genuinely big guy, unlike TT.
In the draft with our 1st I'd take Jalen Johnson or Franz Wagner. Both versatile forwards who can do a bit of everything. Franz is a bit older, can shoot significantly better and has a bit more polish - Johnson has a higher athletic upside and is a significantly better rebounder. Either would be worthwhile in my opinion.
In the second I'd take Herb Jones and Isaiah Todd. Todd can continue to play in the G-League on a two-way (I can't see any reason in retaining a turning 26 y/o Tacko and a 23 y/o Tremont). Jones is a do-it-all wing who can genuinely play 2-4 on defence and can playmake, so I'd be pretty happy with him stepping into that third string role we lose from Grant being shipped to Memphis. Todd was ranked 13 in his high school class, he's an impressive athlete at 6'10 with a 7'0 wingspan, and he's got some nice skills. Can play stretch 5 but is pretty useful as a 4. Worth taking a punt on.
I'd also love to add a veteran like Wayne Ellington to the bench in free agency. Basically only shoots threes now, but with Fournier and PP off the bench taking most of the playmaking duties I'm happy to have someone there to keep the D honest. If Langford develops a three point shot he overtakes him.
So our rotation, ultimately, would be:
Jonas Valanciunas / Robert Williams / Luke Kornet / Isaiah Todd
Jayson Tatum / Jabari Parker / Jalen Johnson / Herb Jones
Jaylen Brown / Evan Fournier / Aaron Nesmith
Marcus Smart / Wayne Ellington / Romeo Langford
Kemba Walker / Payton Pritchard
That would be an ideal off-season for me. I would think that this team would be an improvement over what we have currently, chiefly for the reason that we likely won't be absolutely hammered by illness and injury. Add some toughness up front with JV and some shooting to the bench and I like what we've got!