Ok well the only probablem I can see with this concept is that it is "Dungeons and Dragons"ish, but lets face it, the iconic version of the characters we have today does come from there originally. So anyway here it is....

Allow rangers (at say 20th and 40th, analagous to warrior weapon specs) to pick a favoured enemy. The details I'll leave up to you for game blaance purposes but essentially what I mean is command similar to the specilaize command

<100% 100% 100%>favoured
You have no favoured enemies.
You have 1 favoured enemy to choose.

<100% 100% 100%>favoured orc
You now favour orcs as an enemy.

<100% 100% 100%>favoured
Orcs
You have 0 favoured enemies to choose.

Essentialy this could be used nicely as an RP element aswell as giving the ranger class some of that well needed variablility that other classes have been getting. You never know what his favoured enemy is unless you choose to implement some kind of echo and even then it depends on the way it's implemented. It could be something as simple as a small hit/dam bonus (say 1/10 char level, like warcry)
to semi-legacy type effects with full echoes when fighting.

Obviouly there is strong incentive to choose something common and humans would be a popular choice, I have two ways of dealing this which i'll get back to. The other option is to make it available to choose classes aswell, every village ranger will be choosing mages but really they've always had an RP favoured enemy anyway, it's no secret ragers hate magi.

Now back to my ideas for the implementation. firstly just a small bonus to skills/hit/dam when fighting directly against the enemy. This is the easiest and I think most boring of the options, but even still it would add flavour to the class( I might just add that these apply only in PK situations, there is no bonus to fighitng Orc mobs).
Secondly implement a different ability for each possible race/class. Against invokers it could be a chanced (albeit somewhat limited to avoid area spell similar to scouts) against Minotaurs it could be a programmed ability that goes off whenever they use gore. The echo should reflect (The rangers skill fighitng this particular enemy, his hatred for them, and something unique reflecting the exact effect of the ability. E.g to reflect the minotaur favoured enemy as above.

Minotaur tosses his head and gores you with one of his horns!
Leaping to the side you easily evade the filthy minotaurs gore.
Grabbing it's horn from the side, you pull the dirty beast of balance, sending it crashing to the ground.
.....causing small damage and lag like a missed bash.

Obviously a slightly large echo but you get the idea. Now to compensate for the prevalaence of certain races make it so that some favoured enemy abilites are are more powerful than others the more common the race the weaker the favoured enemy ability. So favoured enemy human would likely be the weakest.

Anyway, there are two ways off implementing this. 1) make the bonus's small and it really won't have that much of an impact on the game at all but would add flavour 2) make them significant and varied, a definate boost in power for rangers true, but one that is relevant only at certain times and with strong RP potential.

I wouldn't have suggested it if i didn't really like the idea, hope you do to, and would love to see it in place if not something similar one day.