If I have worded this correctly, it can copy Legendary creatures without triggering the 'legend rule'. Plus, it steals them for you into the bargain.

This is cleaner wordcount wise but is it better than the original text (reproduced below) ???

You may have ~ ETBs as a copy of any creature on the battlefield, except it is still named ~. Whenever ~ copies a creature, gain control of that creature as long as it is being copied by ~. At the start of your upkeep, you may have ~ become a copy of any creature on the battlefield, except that it is still named ~.

The first ability does indeed work, but it's just the same as Sakashima the Impostor. However, the steal-as-well is fascinating. I'm not sure if the rules currently handle "as long as it is being copied by", but I don't think it should be a problem.

Thanks.

This card is meant to represent a key player in what is going on. SPOILER ALERT

One of these guys is responsible for the decline of the Valurian Empire, having dominated/impersonated the last 5 emperors

I think the flavour's interesting, especially if it's reflecting a specific part of the story. I guess the "as long as its being copied" wording works, although it might be safer to avoid it if there's an easy way to do it. I think it's clearer if the two abilities are combined into one. It may be easier to template if instead of having it retarget, you have it return to hand.

I will keep looking/testing. So far we have had no issues with the "as long as its being copied" text. Not too much different from "As long as....remains tapped" which is where I got the idea from.

I agree it's completely intuitive and will definitely work with the rules 95% of the time; it's just the sort of thing that trips my "this may be ambiguous in weird rules interactions", eg. if it copies something else, does the previous copy effect stop, or merely become irrelevant, and if it's still there, does the current wording end the gain control effect? Or what if, you apply another copy effect from mirrorweave before retargeting? I don't think any of these are fundamental problems (they may already work with the wording as is) and that there's no need to worry about how to template it until you've finished playtesting, but it's something to bear in mind for later.

Added a rider clause reminding that previously copied creatures are no longer copied which should solve some of the ambiguity. Need to find a way to reword this economically as its word count is WAY too high at moment.

Oh no, I'm sorry, please don't add MORE words to the card. I meant it when I said you shouldn't worry about it yet. I'd suggest using "When Dominant Doppelganger enters the battlefield or at the start of your upkeep, you may have Dominant Doppelganger copy any creature on the battlefield, except it is still named Dominant Doppelganger, then gain control of that creature until you copy another creature." or some variation: I think that avoid problems and is reasonably clear, though you'll probably have to reword it at some point.

That wording is indeed shorter and still works. Thanks dude.

That doesn't quite work as it doesn't say what happens when Dominant Doppelganger leaves the battlefield (it certainly will never copy another creature!).

addedc "or Dominant Doppelganger leaves the battlefield." to fix this.