The additions to WoW have been great quality of life changes, but over time it's degraded a lot of the community friendly aspects in favour of making all the servers connected to create a 'small world'. While it's a lot easier to get a random group going, you lose a lot of the personal connection you have with people on your server, and with building meaningful relationships with the people you play with since you're likely to play with a stranger you will never ever see again.
So here goes with some basic ideas.
1. World Bosses/Rare spawns for the casuals. Have something that spawns that multiple people can participate in fighting littered around the world. Have them reward valour or reputation, something valuable that makes it worth while to kill but not necessary for progression. Have these challenges be available for people to get a small group going and scale to the difficulty of the group. It could be done similarly to Scenario bosses, where you might not need a Tank or Healer to kill but it would help to have them.
2. Reputation reinstated for dungeons. Dungeons were a huge part of why I played, and dailies are really horrible. Running for reputation was a much better reward than running them for the daily valour/token rewards. You had a reason to kill every mob and take your time through the dungeon over running it as quickly as possible with an impatient group.
3. Epic group quest chains. I loved TBC when they had some really long quest chains that ended with a boss you had to kill with a group, almost like a mini-outdoor dungeon boss. There was reason to get a few people together to fight the boss.
4. Wintergrasp. It was one of the best outdoor PVP zones created. Even if it had some flaws to the system, it was fun when a lot of people participated just to try to claim the place every week to run the special bosses or gain access to the quests and vendors. Doesn't have to be this location, but create a zone that's always relevant to content or update new areas with their own PVP type zone like Wintergrasp.
5. No-Fly Zones for World PVP. Or just anything to help world PVP along again.