I don't believe that having an "elite playmaker" is a perquisite to a title team, if by this you mean a traditional PG type player vs. a "point forward" or "point center". I get the feeling that this is what is being implied though. That said, it also isn't a bad thing to have a high level playmaking guard.
The two that have been mentioned that would fit this for the Celtics are Garland and LaMelo. Both of these are high level players. I think the idea is to trade Brown for one of these. These are not the types of players that I would typically want to target, I would still prefer a playmaking big, but either of these are definitely interesting.
Either due to talent or contracts, any trade would have to include more coming to the Celtics than just a 1 for 1 for Brown but it does seem like a potential deal is possible. LaMelo in particular is not the type of player I would typically target but if they would trade him for Brown with the idea that they then draft Scoot Henderson, I think that is a very interesting possibility.
Are there any playmaking bigs available?
Two of the best in Jokic and Bam were just in the Finals. D Sabonis is likely unavailable given SAC just had their best season in 15 years and there is a lot of excitement over there about their future.
Draymond is the next guy although I am not sure he would fit in that well with the Jays (slashers, on ball) vs the Splash Bros (shooters, off ball).
Sengun is one guy that gets mentioned in some of these trade proposals. He would be good. I'd like to try and pry him loose with a non-Jaylen trade. I do not feel he is good enough to sacrifice Jaylen for.
The next level down of playmaking bigs like Olynyk or Vucevic. It is more good passing bigs than great passing bigs. Not that different than what we have with Horford.
Towns has been discussed, I guess that was what I was thinking, but your point it fair. There are going to be far fewer options to get a playmaking big. And when I say playmaking big, I am not thinking the player has to be able to bring the ball up the court and make passes like Jokic can do, I just mean a big that can facilitate offense through all aspects, scoring, passing, screening. Will require some attention from the defense, maybe even double teams. Tatum is still always going to be the primary playmaker. This is a question of the secondary playmaker.
The lack of available or gettable playmaking bigs is part of why you start to look at players like Garland and LaMelo. And to be clear, I think Brown is a very good secondary playmaker. Maybe not the ideal secondary playmaker to Tatum, but still very good. A guard, ball-handling type or a big as the secondary playmaker, would diversify our offense some, allow us to apply scoring pressure in different ways. Give the defense more things to defend against. But if we are going to swap Brown for this different secondary playmaker, guard or big or whatever, that player has to be really good.