How would you stop XP trains?

Make the mobs stronger when they're together, if they're from the same family of mobs within a dungeon. And ideally they'd have environmental hazards spawn if their numbers go over some system-based limit.

And mob aggro could track how many people there are within a certain range from them and if there's too many people the aggro range would increase, which would lead to the stuff I mentioned in the first point. This would also make dungeon runthroughs of raid groups a bit harder, because a ton of mobs will be aggroing onto them and those mobs would be stronger and have hazards spawning all around the raid.

Well, at least I'd personally prefer that kind of approach.

Not sure I follow since the games I come from, the devs USUALLY just tweak things until 'the way they intend players to play is also the closest to optimal'.

But I also come from games with:

1. Smaller 'packs' of mobs so it takes quite long to gather them up.
2. Mobs with abilities similar to players so if you try to do this, they use ranged attacks and make themselves difficult to all keep together (for hitting them) without dying
3. AoE damage division after certain numbers of enemies
4. Level difference based damage/accuracy scaling (probably not relevant to this)
5. Healer mobs that would need to be killed separately before pulling the others or when you get them to the target location or they will backline and heal others
6. Mobs that cannot be killed in even five AoE for actually good exp by basically ANY class (this might have changed, but it combines with the first point, you can do it safely but then you don't have enough mobs and might as well just roam and kill them)
7. Mob tagging mechanics that don't allow outside groups to attack your mob, far less 'attack it and you get even proportional EXP for it'
8. Mobs that are generally so difficult to solo FOR GOOD EXP that most people will only target their 'favored prey' or do it for personal challenge and fun, or if they are antisocial.

Once we know which of these things players/Intrepid disagree with (for example, we know that as of current design, we definitely won't have level difference based scaling) we could start to guess at what they intend.

Or we can just abuse anything we find in Alpha-2 and complain about it online.

But to answer the question, I GUESS I'd... do everything from the games I already like? (Shocking, truly)

Except maybe the Mob Tagging mechanics, since Ashes wants to be very specific about that. I'd probably do the usual and give the 'multiplier for party' and then not give it for non-party players.

"Two players who are not in the same party hit the same mob, kill it, get 100 exp total split between them based on 'contribution' with no bonuses."

vs

"Two players in a party hit the same mob, kill it, get 100 x1.2 (as example) exp split between them evenly without contribution factored."

and of course

"Two players in one party and two players in a DIFFERENT party hit the same mob, both groups get (let's say half damage from each) 50 x 1.2 but if they grouped up all as one they'd get 50 x 1.4."

This way if you do WAY more damage than the other party, you can just keep going until they leave due to poor exp without really harming yourself. If you do equal, you should probably group up, maybe fight something harder.

Since this is what I'm used to, this is what I tested in Alpha-1. The mob respawn rate and values were tuned to 'what I expect from games' (it's like 80% of the reason why I'm still here).

The thing that stops high level players from farming low level mobs in FFXI is that they kill them too fast and then have to sit around and wait for them to respawn for worse exp. In a game where Mob Tagging is different and PvE 'griefing' is possible, I'm not sure this would be enough, but for the specific problem mentioned, it should be.