Perhaps you folks might start by answering this important question (something that I've asked in many previous development cycles):
What makes a game Ars Magica?
In other words, what are the game's essential qualities? Are they mechanical, thematic, procedural, aesthetic, or a mix? When does it stop being Ars Magica for you? How much would it not resemble ArM5 while remaining Ars Magica?
And a followup:
Who is ArM 6's audience?