Since we are going to have 3 year keepers, how about this.
One keeper is a 3 year keeper and one is a 2 year keeper. You can trade the 2 year keeper, and if traded then they gain one year of eligibility, so they become a 3 year keeper. If you are unable to trade your #2 keeper before the date of selecting next years keepers then they must go back into the draft, and you have to select a different keeper.
Players that were not keepers this year, but become keepers next year (Dwayne Wade for example) can become a 3 year keeper. At the end of the 3rd year we have to select 2 new keepers (or 1 if we have a Dwayne Wade type who has keeper eligibility status remaining), and all the original keepers go back into the draft. Since we are only keeping 3rd and 4th round keepers, we have a 12 team lottery for the following draft. The lottery would be similar to this year, where you can select your draft position, but instead of 2 groups of 6, we have one group of all 12 teams.
What this does is keep KG and Duncan from getting traded over and over, but does allow all of us to find a #2 keeper that we like for next year. It also doesn't preclude trading Garnett for Duncan, so that Derek and Joe can keep Nash and McGrady, if that is what they want to do, but it does change the complexion of the league a bit.
It does create a situation where Derek could get Garnett again, but he has 1 in 12 shot at it, same as everybody else. Also a 12 team lottery eliminates the possibility of people tanking to get a high draft slot, but it doesn't change the strategy of trading to get a better keeper, albeit a 3rd or 4th round keeper.
Just read Derek's post about moves that have already been made. In the case of Tracy McGrady, he was with Team Sky for year one, and he was a keeper. He was traded during year 2 to Bods Ballers. Based upon what I am saying above he now becomes a 3 year keeper, because of the trade. That means that Derek has his 2 keepers for next year already determined, assuming he doesn't choose to trade one. Trading McGrady again though, won't affect his keeper status as a 3 year keeper.
I don't see any problem with this McGrady situation. What difference does it make if you trade him in season or in the off season?