*Super-mercs*
They are higher-level mercs. They sometimes will have builds not usually found in mercs, such as minion-mancer or rezmer. The trade-off is that they take up 2 or 3 party slots each (depending on level), and they take as many shares of gold and loot. Some may have two professions.
*Super-heroes*
Same as super-mercs, except for heroes. They level up such that they aim to be a higher level than you. You can use them in PvE and some limited areas of PvP (such as special arenas) - depending on popular preference.
*Super-minions*
Same general idea, except that it is minions that the necro can summon that it counts as 2 minions. This includes the cost of spells that affect them, where the cost is based on the number of minions, except that the benefits are not doubled. Smaller army, fiercer soldiers

*Balance & Pros*
1) Just like monsters, you have either larger groups of lower-level monsters, or smaller groups of higher-level monsters, so it is not a balance issue.
2) They take extra shares of loot, so it is not preferrable for farming.
3) They reduce AI and bandwidth demands on the servers.
4) They allow you to play with cool allies.
5) They have higher life and armor, so you become more appetising as targets. It introduces a new dynamic.
*Suggestions*
Hydra.
Bone dragon.
Warlock.
Rollerbeetles.
Trolls.