There were a number of suggestions on the WotC boards about making Rage activate on a trigger. Most of these were 'when bloodied' which is obviously a poor design, given that a player has considerable control over when they are bloodied and can exploit it accordingly. I think an interesting mechanic would be a damage-based threshold though, with some optional player-chosen input to when exactly they trigger Rage. Consider as a starting point the idea of taking at least half of your maximum hitpoints in damage - the simple Barbarian would automatically Rage as soon as this happens (and neatly gain resistance in the process, making the hitpoint loss not that big a deal - indeed I would extend the resistance to all physical forms of damage including magical versions thereof - fire, cold, force, poison and so on). To give the player some control, you could define a range of hitpoints during which you can rage, and beyond which you must rage - I will call this the Rage Range. The low end is half of your maximum hitpoints minus your con modifier times by your level, and the high end is the same but plus your con modifier times by your level. A 5th level Barbarian with 14 Con and average HP has a maximum of 46 HP (12+2+(6+2)*4) and so their Rage Range is when they take between 13 and 33 damage in a single combat - they can choose to Rage as soon as they've taken 13, and must after they have taken 33. The same limitation on the ending of Rage would apply, only I would allow rounds in which you make strength checks to count towards maintaining Rage.