Yeah great ideas, I was thinking something like that. A guild wide bonus or a system where the Guild Leader can give someone/multiple people an active bonus.
It obviously wouldn’t be anything major, but I’d rather see bonuses as opposed to straight up rewards. Because in the end the rewards wouldn’t be enough anyways. It’s just a way to celebrate as a guild and promote guild unity.
I’d also like to see some kind of activity the guild can work towards together and perhaps get a global emblem or bragging right. Just more ideas on guild unity. 
Gold bonus, or soul gain, or xp gain would work for great bonuses as well though. Something that’s fairly usable, but isn’t too substantial.
Imagine doing an targeted activity together as a guild, and working towards getting something like an exclusive guild emblem? That only guilds that work hard together can get.