I'm currently playing the "Together Forever" challenge with a friend, and I had an idea.
Right now, a lot of people—mostly solo players—are using Warmode solely for the XP bonus, only to complain about getting killed by high-level players; this is a major issue, especially on the Hardcore server.
Then I had an idea: why not an alternative with an XP bonus for PvE players that also encourages playing together?
The "Together Forever" challenge would be perfect for that. Without a group, you don't get any kill XP for it, but being in a group offers hardly any advantages—except for getting that kill XP back. The teleport is nice, but the long cooldown means it doesn't get used very often.
Why not add another incentive there? Perhaps a flat percentage XP boost while playing together, or even more XP per player in the challenge—say, 3%. With five players, that would amount to 15%, just like in Warmode.
Of course, that could cause a problem in conjunction with War Mode. People might combine them to gain significantly more XP—unless steps are taken to ensure the effect doesn't stack.
Unless, of course, it might actually benefit open PvP—especially on the PvP server—because it could lead to the formation of groups that engage in genuine PvP wars in the open world.
Just a quick thought: I enjoy playing with friends, and the challenge would be even more appealing if there were an extra incentive. Right now, we just play together so that one person doesn't accidentally gain more XP by grinding while the other is offline. An incentive like bonus XP would be a great motivator for us to play together more often.
Best regards
Darkshooter