I am unsure if there is any reward for the Top 20- 50 guilds but I do not think at the moment there is any.
To encourage guilds to build up and improve consistently and constantly, I would like to suggest a reward system that can be given out at the end of 1 or even every 2 seasons (about 1 month once). This can be in the form of crowns, sps, dec or even gladiator packs. There can be other reward ideas too.
At the point of writing this there are 1361 guilds so reward for the Top 50 or even Top 100 is pretty reasonable in my opinion.
There can also be a condition that these guilds must be at least at Tier 3 minimum or more. This might give more incentive for guilds to move up Tiers and give the lower guilds better competition at the lower Tiers with more guilds with players at about the same levels.