Interesting ideas. For new elements, what if we had light and darkness elements. Just a thought . Also new bosses or so is a cool idea. For the real time co-op play, I can see that working in campaign but do you want with bluetooth, NFC or WiFi or ethernet? it's interesting but it might take up much internets and be big on latency (server lag). I mean DQ has pretty laggy builds, a lot of movement and I'd think you'd get kicked off the floor by disconnection if you get disconnected randomly; that isn't fun. Even I still have mediocre internet at best. It's 2013 old and even then, you can sometimes lag on online game and lose due to it.
In a way, dark element is already arcane but how about demonic and angelic element to fight the game theme.
Friend requests? I mean I guess so but what about chatting in game somehow. Chatting just for shits and giggles . For what reason I don't know though. Real time PvP as cool as it would be, it would be very difficult and probably a thing for a DQ 2.
Friend and you as AI in campaign to gain XP offline, eh idk about that. What's the point in that if it's not you and the friend being engaged but the offline AI.
This would break the core of the first game which was meant to be mostly offline and online is totally optional but fun. The online part is the AI facing real player and searching for real player AI but when you're in the match, you can fight offline 3-0 or 0-3 or draws and then immediately turn on the internet and win or not receive anything because you have no connection.
My open world DQ idea as a mod of Minecraft would probably be a way to experience fighting DQ weapons in real time for the first time or an entirely different game which is inspired by DQ. It doesn't have friends list but you can still hang out with regular friends and fight for fun real time. It's not actually existing yet but if it were, this is how I'd envison if mods work on server gameplay.