One thing I have been considering if I ever get around to running another 3.X game is to let Ranger's get critical hits on enemies who are normally immune to crits if it is their favored enemy. So, a Ranger with Undead as their favored enemy could crit undead, and a Ranger/Rogue multiclass could sneak attack Undead if they had Undead as their favored enemy.
I like it, but I'd grant it somewhere around 5th level.
Lots of builds dip into ranger for the TWF. This ability at level one make the dip even more attractive. Putting it later would prevent that.