I think any team that finishes in the top 5-10 in both offensive and defensive efficiency has a chance to make a deep run and even win a title if lots of things go right (04 Pistons; 11 Mavs; 19 Raptors, etc).
Even if you're top 5 in both categories, if you don't have a clear cut MVP type player, you're probably still a long shot (e.g. Utah this season .... 2015 Hawks ... etc).
My point is that you really can't plan or aim to have that top 5 MVP type player. Unless you're the Lakers or maybe one of the other glamor market teams. It's just so difficult to actually end up with one of those guys that it's pretty much pointless to try do it. I think the best you can do is try to put together a talented, cohesive team that can be elite on one end of the floor and very good to elite on the other end, and then hope to be lucky.
Hope that your playoff matchups break just right. Hope that some calls and crazy shots go in your favor. Or plan to build a very good team and hope that one of the guys you thought was just a very good prospect turns into a league-leading superstar.
I suspect that neither Tatum nor Brown is ever going to be in that LeBron / Curry / Giannis etc tier. But we've already seen they can be your leading scorers on a deep playoff run. We've seen Brad Stevens engineer elite defenses. We've seen years where the Celts were a top 5 offense and top 5 defense (last season was such a year, although that was partly due to Hayward being healthy and on the team during the regular season).
I don't think radical changes to the core formula (Tatum, Brown, Brad Stevens, elite defense) are needed in order for this team to be winning 50+ games and making deep playoff runs again.
If you're title or bust, though, history says you're probably going to be disappointed until the Celts find their way to having a MVP type player on the roster again. If you know a sure-fire way to get a guy like that, you'll be the first person to have figured it out. Tanking is no guarantee. Free agency is no guarantee. Trades are no guarantee. Unless you're the Lakers.