Mounts and companions you get in classic SHOULD NOT be usable across multiple characters on the same classic account, however, you should be able to have these rewards in retail. This will increase the longevity of classic.
While I can fully understand the argument for “people will just play classic for the achievements/mounts/companions instead of classic itself and the people who want to play” that is not in and of itself a bad thing. Some people only still play retail for mounts/companies/appearances such as me. While I would be playing classic for the gameplay, getting things I can use on live as well would be an added bonus and would encourage me to use the servers more.
I can understand the concerns for a slippery slope, which is why I think they should only, solely, and be nothing other than one way. Classic appearances, mounts, companies -> retail.
Unique rewards might be something to consider, such as special sets, pets, or mounts for completing raids and dedicating time in classic. Someone who only wants the rewards and wants the single player experience won’t be able to get those rewards and will be forced to immerse themselves in the community if they do, similarly to how if you wanted to raid you had to immerse yourself in the community. Except now you have raiding and something you want in retail. This will help to solve the problem that’s in the community right now of silence and single player experiences with other people around by reminding them what its like to be part of a realm-wide community in WoW.
This is for the longevity of the game, to make people play both classic and retail. You don’t have to make the items the exact same appearance but something that will make retail players go to classic for an extended period and something that will make those who play classic try retail.
The “people will just do it for appearances/mounts/pokemon” argument is sophistry.
Imagine someone joins a mythic raiding guild with the intent of just getting the mount from the last boss of the tier. They still have to prepare for raids, integrate into the team and follow the raid mechanics, they won’t just /gquit after they get the mount either. How is this any different from that?