What are some things you missed out on in old pokemon games for whatever reason?

Is it worth replaying Emerald for stuff like this? It was my first pokemon game and my cousin showed me a few things like groudon/kyogre and the regis, but I never knew about gym leader rematches, the steven battle, or completed any of the battle frontier. Are those things fun enough to go through the game again and the grinding? I'd probably use an emulator with speedup for the more boring parts.