With the awesome new two-man system maybe nobody would want this?
Can hire only basic mercenaries
Warrior - Punch Skill - 500hp - average attack damage
Tank - Rescue Skill - 650hp - low attack damage
Healer - 4 Heals - 400hp - maybe limit 1? 2?
Mercenaries;
Take a group slot (1 per mercenary)
Cannot be equipped
Cost gold to hire (maybe increasing costs for increasing numbers)
Last until released/rent/dead
Pretty rough, but i think there is some potential there - thoughts?
