yukiAs I've said on many occasions, I wouldn't divide up 'scout roles' like you would soldiers; they are extremely interchangeable. Since you are going to anyway, I'll give 2c.
I find some of the better 'passive scouts' tend to have a lot of initiative, and know when to take the beam and 'lead'. There are those times where, as scout, you have the smallest window to get in and do a lot of work: if you wait for someone else to call it, it's already too late.
If you're gonna play 'passively', don't just sit on the combo and eat the spam. If you're going to do it right, you'll combine some situations where you lead (see above), vs when you sit behind your combo and let them eat the spam/waste the other team's ammo, and you follow up with the mop.
I'm currently maining scout, having mained it last season and playing it a bit in ESEA. In the past, I generally played zero heals, getting medkits, and only getting buffs. In a 30 min game I'd get 250 heals (overheals excluded), which freed up heals for other players, and just suited my playstyle of wanting to hold w. As time went on, I realised it was better to have situations where you 'pocket' (a) scout(s), even if it involves neglecting heals on your heavy classes. This is obviously because scouts reload quickly, have high damage (duh meatshots), and high mobility, making them more favourable to heal in a lot of post uber exchanges. Because of this, doing stuff like counting ammo used and uber %'s (when ubered, i.e. knowing when ubers are fading) can make you a real weapon.
Playing any scout role effectively requires a very good understanding of the game. You'll notice that some of the best scouts have main called previously, and didn't even play scout for that long (zebbo, b4nny, clockwork [didn't maincall but meh]). It's kind of a given, but just thought I'd put that in anyway.
good luck
So let's say my team doesn't want to divide the scouts roles as you advised, how do scouts would know when it's better to go on flanks or when it's better to stay 'passive' as you described it?
From my poor understanding scouts are supposed to do the Flak for the team which more or less implies to stay at all times around the combo, but that means we never really do flank plays (or even hold flank, we just watch it usually)