I would say that combining the two modes won't be as simple as that and that he maybe won't be able to do it right away. However, if its possible and he makes it, with sage mode potencialized by the 9 tails chakra i think he'll get:
- Ultra strenght (think tsunade/sakura strenght VS 1.000)
- Superspeed (body flicker to the extreme since he can use huge amounts of chakra do boost his movements)
- Ultra perception of everything from enemies, to jutsu, to projectiles, to his surrondings, etc. (with sage mode he can sense chakra and see chakra and natural energy; boosted by 9 tails chakra he has the possibility to develop a kind of "omniconscience" of whats arround him, dodging from attacks, countering and previewing jutsu, cancelling and resisting to genjutsu, predicting enemy movements, knowing if the enemy speaks truth or lie, etc etc etc)
- Trowing normal rasengans (he did it with fuuton rasengan, so why won't the next step be doing it with the normal rasengan?)
- More elemental control (he could master new fuuton jutsus and other elements, possibly earth or water since they both could counter fire but are weak against lightning)
- Fox's knowledge of jutsu, madara and the world (if he gets the cooperation from the fox he'll be able to acess it's knowledge and experience as well as some special abilities not yet seen, since all other tailed beasts seem to have an afinity to special abilities and the fox, other from beeing a gigantic fox that shoots chakra spheres, hasn't been shown yet to have a special ability like lava, sludge, fire or sand releases like other beasts)
- He'll perfect "that jutsu" (wich we still don't know what it is)
- Maybe he'll learn some special sealing techniques that might be hiding away in his genetic memory or something.
- He could acess a random power-up like for example gaining rinnegan or other keke gekay that simply's been laying dorment.
Can't wait to see what comes next...