• Getting haste from is questionable.
• There is a probability for memory issues when creating copies, especially here since the token is given a bunch of abilities even if this leaves play.
• It kinda bugs me that this is a Shapeshifter yet in actuality it doesn't itself shapeshift, but creates a clone of another creature.

>Getting haste from is questionable.

No it isn't. It's not common, but I don't see how a card having something to do with a secondary/tertiary part of the color pie is "questionable"

>There is a probability for memory issues when creating copies, especially here since the token is given a bunch of abilities even if this leaves play.

Saheeli's Artistry (which is actually a bigger deal, because it's a lot easier to forget something is an artifact than it is to forget something like flying.)

>It kinda bugs me that this is a Shapeshifter yet in actuality it doesn't itself shapeshift, but creates a clone of another creature.

Actually kinda a reasonable point.

Getting haste from is questionable indeed because the ability tertiary in it. It's pretty far in there and not something it commonly does. So it doesn't represent well when grants lifelink and gives flying - both of which are extremely common place in their respective colors. Haste isn't. So it might as well as be getting that haste from .

While forgetting that something is an artifact might be easier, it's less relevant from gameplay perspective than forgetting lifelink or flying.

The reason for haste is, I don't know what other green keyword would work well here: Hexproof would be too strong here. Trample would be sorta redundant with flying (double evasion, although I do realize some cards do have both, but honestly, that always seemed like an odd design to me) Reach is pointless with flying. Deathtouch: I guess I don't see a huge problem with this one, but I just don't like it here. Vigilance: Then we'd have an issue with all 3 of these keywords being white ones.

>While forgetting that something is an artifact might be easier, it's less relevant from gameplay perspective than forgetting lifelink or flying.

As a rule, I generally design things with the expectation that players can handle things like remembering that something has lifelink. Honestly, I can't imagine anyone having trouble keeping track of something like this. There could even be a token for the clone that has those keywords on it so it's easy to remember.

Okay, I feel ya, bro.

How about ? :

> Pwyll the Second
> Legendary Creature — Shapeshifter (mythic)
> When you cast ~, create a 3/3 Shapeshifter creature token that's white, blue, and green.
> You may have ~ ETB as a copy of any creature on the battlefield except it gains "This creature and creature tokens you control have prowess, trample, and vigilance".
> 3/3

That templating doesn't do any of the things I want or reasons I designed the card. The idea I had in mind was something that encourages making a deck based on populating clone tokens, populating a 3/3 isn't very interesting.

As far as the choice of keywords, I don't think they are bad, but I just like the ones I chose more.