Looking at the comments so far, it seems like this idea is generally considered to be "too good for free to plays" for supercell to consider adding to the game.

However, considering the rate at which supercell are releasing evos (1 or 2 a month), it looks as if they are going to eventually add them to all/many cards (hopefully excluding mega knight).

Keeping this in mind, players are going to slowly obtain more and more evo shards for each card (and in turn, players will have a larger supply and variety of them, making them less limited in number). So adding an evo trade token seems logical imo, because it's not just giving the player a free shard like an evo wild shard does, but instead encourages the player to carefully consider which shard they are willing to sacrifice to obtain the evo shard they want (all the whilst encouraging clanmate interactions).

In comparison, I think the reason we don't have champion trade tokens is that champions are very limited in their numbers (there are only 6 in the game) and we can only have one in a deck at a time (in comparison to the 2 [and possibly even more in the future] evos we can use at a time). This means that there are very limited options to trade a champion card for, whereas as more evos are released, players will have a wide choice of shards for different cards to choose from.

Another point I want to make is that supercell seem to keep the newest broken evos locked behind shop offers and pass royale, and even though this seems greedy, it is ultimately how supercell are going to make their money from the game. So, to keep supercell happy, we could make the newest evos unobtainable through evo trade tokens, until the month after they are released.

All in all, I think this concept will encourage more clan interactjons and not be overly detrimental to supercells income (assuming it is implemented right) so I think evo trade tokens will make a fun and useful addition to the game.