I wouldn't do a trade for Embiid either. I would love to see Simmons on this team, but the difference between Brown and Simmons is fairly neglible at this point, which means trading one for the other probably doesn't make a ton of sense.
If it could be done with Walker, young players and whatever draft picks and swaps, I'm on board (even though I love Kemba). A core of Simmons-Tatum-Brown would be unstoppable. Those guys would magnify one another's strengths and be a ton of fun to watch. If you throw Smart and Ojeleye/Grant Williams into that lineup, I'm not sure any team would ever score on the Celtics.