Guilds should be encouraged to grow, not discouraged.
Guilds are a cornerstone of MMOs. From a developer's perspective, they should be embraced for what they are and shouldn't be chastised unfairly for the negative impacts that they sometimes have on gameplay.
The reality is that Discord (and to a lesser extent Teamspeak and Ventrillo) changed the way that MMO gaming is played. These tools allow players to interact directly, in real time, outside of the limitations of in-game chat/voice. They allow for the creation of communities, friendships and comraderies that span multiple games and years of game-play. They allow for a level of organization/coordination not often planned for by developers.
Naturally, some of these guilds have become very good at what they do and as a result they tend to attract quality players and frequently dominate gameplay across games.
Recognizing this, it falls to the game designers and developers to come up with ways to find some balance between large-guild and small-guild gameplay. That said, it should not be a goal to dismantle or even to discourage large guild game-play.
In most MMOs that I have played, in truth, I have been disappointed with the developers' approach to guilds. Very little effort, I find, is put into contributing to the health, well-being and pride of these guilds by game designers. I find this to be very surprising given that these guilds often form the lifeblood of these games. It is often these guilds that take server-wide leadership in promoting/creating content when the game itself struggles for new content, often for years on end. Yet, what does the guild gain from its long-standing support?
Trying to discourage the formation of large guilds is a non-starter. They already exist and their communities are extensive. They are successful for a reason. These guilds will adapt to in-game mechanics that are designed to benefit small guilds. Limited number of player buffs per guild, for example, will simply result in the creation of sub-guilds. Discord enables the organization and coordination that will still make them successful.
Yes, large guilds have an advantage of numbers and resources, but large guilds also have to deal with a myriad of large guild issues. That's why not all large guilds are successful and why a number of smaller guilds are quite effective.
In either case, I believe game designers should find ways to help promote guilds that are tied to the success of those guilds in-game as measured by its member contributions (both individual achievement as well as guild-specific contributions). Statues honoring guild-firsts... achievement rankings by guild (adjusted for player number), guild titles, node-naming rights, castle naming rights, guild progression paths, guild banners, guild storage sizes and adornments of varied size and status, in addition to guild buffs. Many of these shouldn't be static though.
In a player-driven system, if guild members leave a guild or become inactive for a lengthy period of time, the reputation of the guild should decline commensurately. If all players had an individual guild achievement score, this could be aggregated and averaged or normalized to a certain size for the guild itself. Guild achievements then become individual member achievements from a scoring perspective before it is aggregated into the guild itself. If a guild member leaves a guild, the player itself should lose a material percentage of their guild reputation score (say 30%) and their contribution to the next guild will be lower upon joining. Yes, high achievement players will still be coveted, particularly those who have demonstrated considerable long-standing loyalty to guilds, but leaving a guild should not be without in-game consequence either. Guild flipping shouldn't be a thing. Having a personal-use guild bank shouldn't be a thing.
The point is to encourage guild gameplay and make in-game guilds a living, breathing player-driven element of the game itself while supporting the fact that guilds are their own communities.
Think of all the things that you wished guilds might have had in all those games that you have played. How did those games fall short? What improvements could have been made? The sky is the limit for AOC...
