I do believe we already have seen the "colorful mage"'s new mechanic.
In the post about the new teaser, many people pointed out something along the lines of "imperfect mimicry". People are thinking that we'll get something along the lines of a Chameleon that can't completely copy it's opponents.
However, other people have stated that the amount of work needed to implement such a thing would be too much for Riot to handle.
The thing is......they've already done something like that.
I'd like to take you all the back to 3 and a half years ago, when the Black Market Brawlers gamemode came out. In that game, we had a very unique item called "Trickster's Glass".
Here is the item's description, thanks to the League Wiki:
UNIQUE – DISGUISE: Teleports to the target allied champion (550 range) and takes on their appearance for 30 seconds. The disguise includes the target's current health, mana and emotes. The disguise is broken upon casting a spell or attacking, or upon your or the disguise's health being dropped to 0. 90 second cooldown.
Here is a montage of the item:
https://www.youtube.com/watch?v=j5b0JeCO9jw
As you can see, we've already seen a copy mechanic introduced into the game, so, the technology for such a mechanic already exists. However, that item copied another player perfectly. There wasn't any imperfections to the copying, which the new champ seems to do.
However, doing such a thing shouldn't be too hard to implement. Maybe just put in kind of a shimmer when the champ has copied someonem similar to what happens to invis champs when they get damaged. Or, as we saw in the trailer, have this champ's tail stick out when it's copied someone (though, that would kind of ruin the illusion).
But yeah, don't think the tech for such a mechanic doesn't already exist......cause Riot made that over 3 years ago.