just playing with the idea here but thought about tactics and its lack of use a lot, especially in my group as we do our own tactics or not at all, it is seen by our GM and perhaps the rules as well as a aid for players who need a bit of help. I understand other players and groups might use it more than our group, but also believe that a lot of others do not use it much or at all.
I play an Ultramarine Tactical marine so I have a lot of options and fluff that suggest getting Tactics, but no use in game and while I probably won't get any use of this idea myself, I figured I'd put it out here and see if anyone else likes it.
The idea is that a tactical person, with appropriate knowledge, would know how his enemy would act and how he should react to this, so would get a initiative bonus.
If you're fighting some odd alien race no one heard of you probably wouldn't know how they fight and thus no bonuses, also if you fight orcs or eldar and don't have the lore for that, you might not get a bonus either. If you never fought Tyranids you wouldn't be prepared for the massive overwhelming horde and how they charge without care (as an example) and wouldn't get a bonus.
If you know these things however you would know where to strike, where they would come and so on.
In melee with an orc you would know he's gonna go all out attacking most likely, so you know you don't have to wait to see what he's gonna do, you can just attack him first.
So basically some kind of initiative bonus, possibly to hit bonuses and such as well, for a high tactics skill. Nothing game breaking, but perhaps a +1 initiative and/or a +5 to hit. Or, use degrees of success so that for every 2 DoS you get another +1? This would be too powerful though but perhaps the idea could be polished a bit and put into use, up to you guys