Cross-Realm Zones: you see people from OTHER realms in your world. (will not be in Classic)
Phasing: some modern quests are able to change the world. If your friend didn't complete the quest, the world is different for him and you cannot see him in the quest area, even when in a group. (will not be in Classic)
Sharding: there are multiple "instances" of the zone, where only players of your own realm are. You cannot see everyone, and if you join a group from a different instance, you fade away from your current instance. (this is being considered as an option for the Classic launch)
If you don't understand why Blizzard is considering sharding, you won't be able to convince them to use something else. I see arguments such as these:
Sharding is used to appease retail players, which cannot stand being stuck a little.
Blizzard wants sharding to reduce the load, but if they could launch in 2004 and the game survived, why not now with better computers? They will use it in the AQ event, I am telling you.
Those are NOT the reasons. Take your time, read, and give this some thought.
This is a famous graph of the number of subscriptions up to WoD, when they stopped sharing it: http://i.kinja-img.com/gawker-media/image/upload/t_original/1241945255815773287.png
Since Cataclysm, the number of players jumps up at the start of an expansion, and then goes down. In WoD/Legion/BfA it was even uglier according to unnoficial reports. BfA subs are already down half. The modern game "survives" because there are all those cross-realm technologies and people don't care too much about the open world anymore.
Now consider the launch of Classic WoW. It only demands a WoW subscription, so it will be cheap to just go in and get a nostalgia shot, and also every current BfA player is able to take a quick look, reach maybe level 8-10 and be done with it. In 2004 the game was the hot new thing... someone buying it to just "check it out" was an outlier.
There is a very high chance 80% of the population will be gone in the first month.
Now how can you deal with this? The current known options:
1) Create many 3K population realms and then merge servers when the populations dwindle. This can be very problematic, because players will lose their character names, and possibly guild names; guild progressions will be mixed; also economics. And this may happen more than once for a player.
2) Create fewer very high population realms (10K+). After "tourists" leave, there will still be a healthy population. Launch, however, would be a mess. Dynamic spawn rates would need to be implemented, and those can be exploited with a huge population. Also we don't know how bad the performance would be with the Legion code.
3) Create fewer very high population realms and use sharding in the starting zones for a few weeks. Sharding fixes the launch problem in the case of huge populations, but creates the multiple instance / players disappearing problem.
Considering the available options, 3) is the least bad for me. There is something that worries me about 2) and 3), however. What if the realm population doesn't decrease? Would we end up with a realm with tons of players and lose that sense of community, of "knowing everyone"?
If you absolutely think sharding cannot be implemented, address the problem correctly. Also try to think of new ideas, guys. "Just repeat 2004" can be very bad for the game.