Reading the comments on the initial PR for NIP-25, https://github.com/nostr-protocol/nips/pull/26 , inspired some thoughts.
Prefixing reactions with the "+" or "-" would allow for multiple emoji reactions to be sent at once, in the form of "+👀❤️🔥". This would allow multiple reactions like on GitHub or Discord, without overly spamming relays.
Supporting the exact same UX as the aforementioned apps, however, might require use of a different event kind, perhaps a NIP-33 Parameterized Replaceable Event. Backwards compatibility could be maintained by sending/storing both the new kind and a basic kind 7 "+" or "-".