My dream scenario is the Celtics keep this years and next years Brooklyn picks. Sign a big free agent this, and get Isaiah on a shorter 3 year deal (big money, of course) when his time comes. This would allow the free agent, Horford, and IT to all be on one timeframe, and the young group to be on another.
Could also set IT up for another big contract in his early 30's so he might go for it. Not sure what year he would be in at that point, but I know you are eligible for larger contracts after X amount of years.
Short term and long term.