or Lotro or other "full featured" mmorpgs. I don't mean limited pets I mean full blown henchmen with capabilities equivalent to those of a player character, something along the lines of Guld Wars Heroes.
Henchmen open up a lot of new gameplay possibilties and they solve an awful lot of Looking for Group problems. Henchmen make a game much more solo / small group friendly. I know a lot of old-timers shudder at the thought of playing an mmo solo but Wow and Guild Wars have shown that making a game solo friendly can lead to commercial success. Given this commercial reality I am amazed that more games have not implemented henchmen. If I were designing a game to dethrone WOW I would certainly try and include henchmen.
Getting the AI right is always going to be tricky but I think Guild Wars has solved a lot of those issues and may offer a template that can be copied. When Guild wars was released its original hencmen were lower level characters with limted player control and very poor ai (melee healers ftw). Updates over the years have greatly improved henchem AI and given the player more control over them. Heroes (a kind of uber henchman) are full characters that level up in their own right and have access to all the skills and capabilities of a player with some minor gear limitations.
Of course Guild wars characters only have eight skills on their bar at any one time so it is feasible to micromanage your main and a hencman or two. Most mmos have far more skills to juggle and this makes controlling a henchman more difficult. However I have always been surprised at the number of people who double (or even triple) box mmos playing multiple characters on multiple computers. I have asked a few of them how they do it and the trick is generally to simplify the controls - putting chracters on follow and binding important skills (for example a healers main heal skill) to easily accessed keys.
Here is a model I think would work for a more traditional MMO like Wow or Lotro or EQ2: Create a number of generic henchmen to cover the main roles: Tank, Heal, DPS, Crowd Control. Make them as powerful as player characters but give them a simplified skill set that covers the main functions of that archetype. Each player can have a maximum of one henchman of level equivalent to their own. Hencham AI would allow for the ususal options (follow/stay put, aggressive / defensive/passive). Players can also take more direct control of hencmen - selecting their targets and using individual skills. Skills can be individually put under AI control or not.
I really think that properly designed henchmen could be usable all the way up to Raid instances with appropriate player intervention. Of course the AI wouldn't be smart enough to tank a major boss but the player controlling it could be.
How do we handle the impact of gear level in gear centric mmos? A few ways. Perhaps players can gear up henchmen from their own inventory. Perhaps Henchmen with different levels of gear can be hired for different fees. Perhaps some kind of gear equivalence could be worked out so that the henchmans gear level reflects the gear level of the player doing the hiring.
What about loot distribution? Guild wars gives henchmen an equal share of the loot and it works surprisingly well. Of course henchmen won't be part of any loot distribution system so they will probably roll on everything. People may be upset to see an epic drop going into the henchman's back pocket but thats the deal if you need to hire one of these guys. An alternative approach would be to give the hencman no loot but pay them a fixed fee depending on the henchman's level and the mission dificulty.
Would henchmen destroy the game by discouraging grouping. I don't think so. Since each payer can only have one henchman you still need a bunch of players to tackle harder missions. The correct use of henchmen could even train people in to playing with tanks / healers etc. I can also see a rich stream of new game play possiblities as people try to work throuhg difficult missions with various types of henchmen.