Assuming the pokedex will allow it in earlier games, can you trade pokemon eggs? I ask because this would involve buying a second handheld, copy of the game, some cables and so on. Few examples:

In gen2 I want a Skarmory/Pikachu early on and both joined so late in the game they never felt like a part of my team

In gen3 I want a Staryu/Onyx in a leaf run, have a copy of red/emerald and breeding unlocked