I have been stewing on this since the off season. I am looking at it from two perspectives though. One is the more obvious or more widely noted and that is the need for a bench big (PF or C) or a 3rd big behind Horford. It is a real narrow needle to thread. To me, just being a little better than Kornet isn't good enough to make a difference. Kornet played 804 minutes in the regular season over 69 of 82 games but only 32 minutes in the playoffs (8 of 20 games). So this added player needs to be good enough to actually play in the playoffs but still "gettable" with the $6.2M TPE. Achiuwa is an example of a player that may be "gettable" with the TPE and a pick but I see him as on the bubble as to good enough to be a playoff regular. Stewart from DET was another decent target but he signed an extension so would be a base year guy and no longer possible for us to get I don't think.
The other way that I have been looking at this is starting by considering that our current starting 5, although made up of all excellent players, may not have the ideal balance. Some view Brown and Tatum as overlapping but I am fine with that now with the addition of Holiday (ballhandler) and Porzingis (skilled big). I see the greater overlap being between Holiday and White and the lacking being a defensive, rebounding type big. So if we could magically bring is a starting level big, say even trade White for someone, who could that player be? To me, someone like Adebayo would be perfect but obviously we are not getting him.
Another option is to consider trading Brown (in the off season), if you are in the camp that Brown and Tatum are overlapping and less than ideal. So who would that ideal 5th starter be that you might be able to get by trading Brown? Brown is not going to bring back Jokic or someone like that, but there are a lot of top players he probably could bring back.
I am sure some are going to say the "small" line up is just fine. I don't disagree necessarily, but I think it is pretty clear that it is not ideal.