Some ideas:
1) Time requirement to open it: the guild has to be open since at least 3 months;
2) Guild need to have at least 50 member;
3) 1 m to buy the "house";
4) 50k monthly rent;
5) lot of lot of lot of lot of stuff to buy, both in game (add a "guild house seller") or from shop. Price can go from 25k (chairs, little things) to 100k (pc to not use pokecenter, tv, other nice cosmetics) to 200k (hire people, like weekly seller, or permanent healer), to 300k (light the first time, or you need to use flash to see lol, add more room, etc.).
6) add a guild house entrance in many places of the 3 region.
Money sink, new donation, not tons of guild, people will be happy. Win-win.