Conversation system (multiplayer)- Keep the text above a non-playable or playable character (his lines) onscreen longer or until the player clicks on Continue or selects an answer.
OR
- If both players are close to a character talking, put the text on the usual dialogue box or a similar window.
- Let the player able to display the combat log (dialogues) during a conversation between his character and his/her companion
HUD- Larger texts for descriptions of skills from the hotbar
- Larger texts for the combat log (or at least conversations)
- Larger portraits in the dialogue box
Avatar- More various portraits
- More muscles on the female Source-Hunter (at least if she's a warrior :p )
In conclusion, improve the co-op dialogue.

It is difficult to understand the storyline or the quests when texts keep coming and we don't know what the character played by the other player said. But I know you're working on that.