I definitely do not want it to be "o hai, why don't i just start following you and pretend we are the bestest friends eva". I also don't want it to be too hard, as I fear it may become distracting/tedious. Something like, complete a set of quests for each companion before they can join. Each companion would have quests catered to its class. So, if you want a barb to join your party, you have to help him/her kill bad guys, if you want thief you have to sneak/steal for his/her quests, etc.