I dont see that happening
Luffy already has the greatest form of Haki why would he need another DF? Luffy perfectly fine with the fruit he currently has and i still can see him further developing his DF powers even more so (like his Gears )
I would also like Luffy to develop his rubber Df but not with gears, they are like doping i.e. the gears are bad for his health they shorten his life span, because they put a lot of pressure on his heart. This is according to Luci of CP9. Luffy needs to have real permanent level ups, not techniques that give him a temporary boost like his gear 2 and 3, which are also bad for health. Temporary boosts are not real strength and they are not enough against truly strong people especially like BB, the admirals and Yonku as we have seen so far in the manga. Luffy just barely won against Luci of the CP9 and he needed help against Moria of the Shichibukai.
He needs to develop his overall stats, like strength, speed, endurance etc..
Rubber gives him a great advantage in physical combat, but that is only for people who are his strength or lower not against an opponent who is stronger or an opponent that has haki or other similar abilities. Rubber Df is not one of the stronger DF and also looks plain in comparison to some of the Logia powers; because of that luffy is forced to use his Rubber abilities in creative ways, but he is coming to a limit as to what techniques he can use his DF. Luffy looks pathetic against some of the Admirals, or some of the other strong pirates out their. Luffy also has been using similar techniques, and these techniques get boring after 500+ chapters. For example some of his techniques are upgrades of his punches.
What does everyone think about this:
First of all, Haki MAKES the punches/kicks harder, so his next power-up will just be to master haki. Regardless, here's an idea for another power-up:
4th gear - what happens when you rub rubber with carpet or something similar... you generate static... electricity... so Luffy has gear 2, that also shoots an electric shock
I like your idea a lot, I didn't think of that, if he does than his rubber abilities wouldn't be boring any more and then he can have techniques that rival some of the strongest powers out there.
It would be good if had a super powerful technique called the GEARS OF WAR, LOL.

similar name to the xbox 360 game. No seriously, may he can develop a technique that allows him to shoot out powerful static electric shock waves? similar to using like an energy wave as big as an island?
http://www.onemanga.com/One_Piece/577/12-13/
http://www.onemanga.com/One_Piece/577/14-15/
BB has been shown to hold the power of the gura gura fruit in both hands, I don't think there are limitations as to what power he can use with each hand.
I have to agree with, I don't think Oda gave BB any weaknesses, that WB did not have with the Gura Gura fruit. I think BB has the full use of another DF with out any extra disadvantages.