Whatever it takes!!!
Well, they dont have to create an exact window.. Whoever is free can come and play and otherwise, the franchises will have to make do with their Indian players.. It would still be a higher standard simply because you will now have 8 teams instead of the 32 in the Ranji Trophy and matches will also have "some" meaning because of the IPL T20 connection..If at current rates a pitiable 900 USD per match, less than what Dhoni made per ball last IPL.
BTW, Your idea of IPL sides playing Duleep trophy could not be practical as a window needs to be found to accomodate the international players. The ICC would never be willing to bend to that extent i think.
As to legends like Warne or McGrath playing the domestic circuit sounds fantastic, like the IPL they can be offered additional mentoring roles like bowling coach, etc, and finances shouldn't be a problem, with a sponsor thrown in. But i would love them at the Academy for youngsters, as the net for catch will be broader.
As to the Irani trophy, I think instead of Asnodkar and tried-and-discarded Jaffer, Shikar Dhawan and a much improved Akash Chopra be given a chance. Also Kaif be considered a look in.
I honestly don't see why they can't try it out...