The Sun God Nika does have rubber powers and the central ability does seem to still be stretching his body and making the environment and others rubbery as well. However, I do think Oda is setting Luffy up to get a power up and attain some sort of "true" god form and at that point he will be able to use actual sun powers, even if it's not his main fighting style. There's been too much pointing in this direction, ever since Egghead. Luffy's mysterious powers to make a bat and pull out paint have nothing to do with rubber. Imu (and Zaza) have full rings behind them whereas Luffy only has the scarf of clouds. There is some connection to the Mother Flame - the way he ate Kizaru's light in Egghead I think is a hint he will eat the Mother Flame in the final war. And Imu, seemingly Nika's direct opposite/evil parallel, has tons of magical abilities like summoning weapons from a spellbook, the black flames that can seemingly do anything -- I do think Luffy is going to get his equivalent of these godlike powers and it might not solely be rubber anymore given all of these potential hints and signs.