Ryan had considered that, and that was why he picked the Tamer class. Fortunately, he got lucky and obtained the skills that would help him in the task of obtaining control of several other domains. The only real issue was that in the end, he could only have so many tamed monsters. So, he wouldn't even be able to obtain control over all dungeons in the country.
"We need to select the right ones because humans still need to become stronger," Ryan rubbed his chin thoughtfully. "While there will always be some dungeons that I won't be able to obtain control, only those can't be used to increase our strength."
Furball was a partner that Ryan couldn't let go of, so he wasn't going to use him for that. The first obvious candidate would be the steel eagle. While the creature could help Ryan save some mana, that skill wasn't that necessary now that Ryan could use Teleport.