In D2 once the expansion hit, you had the option to make each character upgradeable to LoD. I kept a few characters in the standard D2 so i could play with friends who hadn't bought LoD.
The only problem I see with this is the shared stash in D3. Maybe characters not upgraded could not use items/loot from RoS. Don't know. I actually hadn't even thought about this scenario because I just assumed it would the same way D2 did. Probably won't though.
this isn't what hes asking though, hes asking for all the bonuses of RoS (new legendaries, bounties, rifts, adventure mode, crusader etc.) but with lv 60 remaining the max cap so that he doesn't have to re-gear.