Trying to avoid outright saying “never ever going to happen”, but let’s discuss your analogy to PoGo.
– PoGo and GoW are two entirely different genres of games.
PoGo is an action RPG-lite, whose central premise revolves around trading pokemon to “collect them all”. Yes, there are rare and powerful Legendary pokemon, but none of them are meta defining. They are nice to have and are a status symbol, but a player can complete endgame raids just fine in PoGo without a Kyogre/Groudon/Mewtwo/Rayquaza/etc.
GoW (especially post patch 3.1) is a gacha game. One of the core foundational pillars of gacha games is the restriction that players cannot trade units with others, by intentional design. The primary gameplay mechanism in these style of games is to induce players to spend (and purchase with real-world money) in-game currency for lottery rolls for units. Desirable meta units are almost always extraordinarily difficult to pull by intentional design to encourage additional player spending. Trading with others who have spare copies of a meta unit would completely undermine this foundational principle of gacha games. Why spend when a player can just trade?
To that extent, there is already a workaround for acquiring Legendaries and Mythics in GoW --> the Soulforge. That’s one of the key reasons why the Soulforge exists. Want to buy these troops faster? Then concentrate your gem income in-game on purchasing the daily offer for diamonds to accelerate reaching the 800 diamonds needed for a Legendary or 4000 diamonds for a mythic. In a decent guild, 800 diamonds does not that long to acquire in combination with purchasing daily gem offers for bonus diamonds. Mythics can be achieved within your 60-day timeframe as well.
Other gacha style games that offer trading generally limit trading with others to low-rarity units only or for higher-rarity units can only be traded back to the game itself for an extraordinarily steep price. For example in Puzzle and Dragons, a player can outright trade for a shiny new mythic (to use GoW’s terms for simplicity’s sake). But, to do so, the game demands that the player surrenders five meta mythics of the devs’ chosing to bypass the need to roll against very poor odds. The offer is punishingly bad on purpose, but it is there if someone really wants to stomach the incredibly steep cost if there is a unit that the player wants bad enough (or has pockets deep enough to possess many dupes of current meta units).
The GoW Soulforge system is much more user-friendly, in my opinion.
And at the end of the day, to be honest, if the devs wanted to power creep mythic availability to be more common in general than it currently is in the game, I don’t believe that they would implement a trading system amongst players. Instead, they would take the much more cost-effective route of increasing the availability of Diamonds to craft Legendary/Mythics and increase the number of each type that shows up each week in the Soulforge.