There were two posts about this already, but one was closed and one no longer exists.
I'm surprised that this isn't already an option. 50 emojis is not a lot. It would be nice if it was increased for everyone, but realistically it would be completely reasonable to have a paid option to increase the cap.
Two options, or both could coexist.
1. Nitro members can add an additional 50 emojis or so to each server they are in.
2. Separate from Nitro, an admin can pay X amount for a specific server to have an unlimited cap. Maybe some other benefits as well if you're going to go this route, where the whole server is upgraded to "gold" level.
Especially given some emojis can be multi-part, having an ability to increase the cap would be huge. It would encourage a lot of people to get Nitro, or to do the "gold server" upgrade, imo.