Evolutions that require trading were a cool concept in Gen I but have overstayed their welcome. Nowadays its only a formality (I send you Graveler then send him back / I trade between my two devices) or an impossibility (for players who dont have online or play singleplayer only)
Each game should be required to have an NPC trainer who will trade you a Pokemon and then trade it right back. However, to balance the game this NPC should only be accessible after the fifth or sixth gym, and might also require an optional sidequest to encounter
This way really Pokemon like Alakazam are still readily available. Every game has a lifespan whether you know it or not. Every game's online mode gets removed eventually, and an addition like this would help to ensure that players can continue evolving ALL of their favorite Pokemon even when they're forced to play singleplayer
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