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I think this would be very useful to know exactly what Pokemon will be used as the disguise when Illusion is in play with Zoroark, so that I know what it will show up as in advance. Probably not a super complicated change but it would be useful!
That's true, but not everyone really knows how it works to the fullest, and maybe just a little text under the tera type or next to the ability saying which party member it will be disguised as would be helpful to prevent the confusion.
There are many interactions that require understanding how a mechanic works to effectively use it in battle. Knowing that Illusion disguises the user as the last member of the party is just something to learn, same as dozens of other mechanics interactions. It is also extremely easy to verify, unlike, say, calculating Eruption's current base power.