Since the Guild Leader owns the Guild and the members doesn't have the right to gain ownership without his expressed permission, you basically have two choices. Also it doesn't matter as to why the Guild Leader is inactive, but there's always a chance of him coming back, he might be sick and unable to play or in the process of moving and doesn't have access to the Internet. Whether s/he is gone for days, weeks, months or even years doesn't matter since the ownership always remains with the Guild Leader.
- You can stay in the current guild and continue doing dungeons/pvp/raids without a Guild Leader. This way you can be in a Guild whose name is desired along with the reputation it brings.
- You leave and move over as many members as you can to the new Guild. Slowly but surely create a reputation for the Guild and its members. The Guild as a hole makes the reputation, not the Guild Leader. But the Guild Leader always retains ownership of the Guild.