I more than once thought about making a party system for mobs so that MVPs don't just summon slaves, but that they are actually partied in a sesne and that they could get customized to have party-wide buffs like Magnificant or Gloria. You could only imagine the rape that would ensue should Alpoccio and Trentini Partied up to do Loki on the players while their mob slaves buffed up and chewed through the desperate players as they cried for mercy.
The extension of this idea was making the server create "dummy" accounts that would have preset characters that could be instanced in some form to be placed in player-owned parties (or they could just group up and party each other) so that they could get buffs and work as a unit. The intended goal for that was to artifically increase the amount of people that took part in WoE battles either in small population servers or in large ones to make it look like an actual war. This would mean that these mobs would resemble actual player characters for all intents and purposes, and they would have gears of their own and all that. And finally, they would have a set of skills that they would do, depending on the job class and the AI type that the player chose.
Finally, to make sure balance is maintained, the number of these "NPC/AI players" (I couldn't really call them Mercenaries because of how fundamentally different they are) would be made proportional to either: a)the amount of online guild members, b)the level of the castle's economy, c)a fixed amount that you can reach, or d)a combination of the above . This final part is more related to giving players a chance to have their castle still be defended in the event that player couldn't come online for whatever reason (not like attacking guilds wouldn't have them in the first place, just not in the same quantities).
They would also be limited in what things they would do and would not do. For example, a limitation would be to cap their level and stats to a point lower than players, to make actuall players stand out as the "hero units". Another thing that these AI players would be limited in is only aiding the players, these mobs would never directly target the Emperium or Prevent an ecall from happening. They would also not target the guardians, and instead would flee from them (or maybe the guardian system could be replaced with this "AI player" system... idk...). in PvE, these mobs would not gain EXP for the player unless the player deals more than at least 60% damage to the mobs and they would also not give drops to the players (the loots they get are discarded form the server). Finally, they would never target MVPs, and instead would go into "flee" mode or "support" mode to just support the player if they can buff or heal.
Meh, I don't think I'll ever get to see that or make it myself since I don't know much of coding, and what I did know of coding has long since been forgotten due to lack of constant practice.