I've been saying for awhile that I though McCollum made the most sense, so I'm sticking with him.
If we could get Siakam or Sabonis without Brown or Tatum, positionally I'd prefer them to McCollum. I'd also be very interested in Simmons, but I think he takes Brown and I'm not sure Boston should do that, unless they get another scorer elsewhere.
I think something like these trades, would be my pseudo-realistic fantasy
It works as a 3 team trade or 2 separate trades
Boston - Simmons, McCollum, Snell
Portland - Smart, Hernangomez, Nesmith, Langford, 22 1st, 24 1st (top 4)
Philadelphia - Brown
So post-trade(s) with 2 open spots
Starters - Schroder, McCollum, Tatum, Simmons, Horford
Playoff Rotation - Richardson, Grant, Rob
Reg. Season Rotation - Pritchard, Snell, Freedom
Deep Bench - Parker, Fernando
That is a more cohesive and better team than what we have and I do think the passing would be incredible around Tatum with that lineup and you don't lose a ton defensively (even with McCollum) given just how good and versatile a defender Simmons is. Aside from dealing with whatever mental issues Simmons has, the only real downside to that is putting Brown next to Embiid. That would make the Sixers a more formidable team, but right now Boston isn't a contender and I do think those collective trades make Boston one, so I do it (and I do think Simmons makes more sense than Brown if you were to just trade for McCollum).