I would probably level in expert mode and rotate through which acts have bonus bounties. Pretty much keep doing bounties until you get enough gear to do torment 1. Then maybe start rifting in torment 1 for awhile. After you get some greater rift keys do some grs to get some gems to use. After you get the gems you want, probably just do bounties and rifts. Being able to go higher really depends on how lucky you are at finding your set. Obviously once you get to torment 1 you want to stay in T1+ so you can get set drops. It is also beneficial to group with people that are the same class as you so you can loot share.
I would probably level in expert mode and rotate through which acts have bonus bounties. Pretty much keep doing bounties until you get enough gear to do torment 1. Then maybe start rifting in torment 1 for awhile. After you get some greater rift keys do some grs to get some gems to use. After you get the gems you want, probably just do bounties and rifts. Being able to go higher really depends on how lucky you are at finding your set. Obviously once you get to torment 1 you want to stay in T1+ so you can get set drops. It is also beneficial to group with people that are the same class as you so you can loot share.
Edit: I guess people are saying just do rifts.