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.