humble minion
Legend
I've seen that done many times in 3rd party products. Kobold Press does a bit of it, for instance. I suppose the main exploit case is using multiple Counterspells in one round and completely shut down an encounter with magic-heavy enemies. But often i've seen restrictions on what you can use for the extra reaction as a balancing factor. Plus of course if we're talking additional reactions as a martial class thing, you'd need to do some heavy multiclassing to get there.Maybe the extra reactions could be a subclass feature?