I know everyone now knows that the Harden trade was a huge mistake (probably massive understatement) but I actually wonder what if you could go back in time and resign Harden...but then actually trade away Westbrook.
The Harden trade was obviously to save money and undervaluing Harden's potential, but if they actually kept Harden, in the short term it may have been beneficial (although I am still not convinced they would have beaten the Heat) in the long term I think trading away Westbrook could have saved Scott Brook's job, kept Durant from leaving and saved the OKC franchise from the mess they are now in. Allowing Harden to start and assume the ball handling role would have allowed him to development into the player he is today (perhaps even better) and as we've seen in Houston, Harden is much better at sharing the ball around than Westbrook and at the same time much less enamoured with putting up personal stats, this keeps his team mates much more engaged and this includes Durant. With Westbrook gone, Durant would have become the sudo alpha and with the Thunder becoming 'his team' would have enjoyed playing for Oklahoma more and would have been almost a certainty to resign for the long term. Trading Westbrook at that time would have brought in a treasure trove of assets that could have pushed Thunder towards a title, be it bench depth, a fourth star or a top draft pick the return would have been much higher than an expiring 6th man Harden.
If you think where OKC were in 2010 to now, there is actually a good chance that Paul George will walk and leave the hollow remains of Carmelo Anthony and a selfish stat padder in Westbrook who may be forever stuck in NBA purgatory, unwilling to change.