Given the enormous success of skins in other games like LoL, Dota2 and CS:GO and their integral role in increasing popularity for thier games (http://www.pcgamer.com/how-400-virtual-knives-saved-counter-strike/), many people have advoated for the implementation of skins into SC2.
There are currently three problems:
1 - Technology.
In CS:GO there are at most 30 models that can be skinned (10 guns, 10 pistols, 10 knives). In LoL, 10 heroes. In dota 2, each ero can be modded with different sets and couriers. The problem with SC2 is that in a general game, it can be anywhere from 200-500 or models to alter. And then you double or quadruple that in team games. Presumably this causes a strain and creates performance issues for the game.
2 - Identity
The second problem is the inherent difference of identity between SC2 and CS:GO/LoL/Dota2. In those games you control a “hero” unit that represents who you are in the game. In SC2 you are a commander of an army and there is less personal investment as you send your soldiers to get slaughtered by the hundreds. Of course this is probably the smallest problem of the three.
3 - Market/incentives
Part of the popularity of skins in games like Dota2/CS:GO is that there is an entire economy built around buying, selling and trading skins. It isn’t enough to just buy/sell skins, but you’d need to also have an infrastructure to allow it, which requires resources and time.
Limit the amount of skins a player can put on into a game. For instance, let there be a loadout screen where they can equip 10 units to have their bought skin. This avoids the performance issue as there won’t be as much slow down and it makes those units “special” creating an identity for the player. Another option is to tie it to unit rank. After a certain amount of kills, the unit is “promoted” in both rank and appearance (Though I can’t speak on the feasibility of this one.)
Random aside, but I’d like there to be esports related achievement/portraits. for instance, if I ragequit 4 games in a row without typing GG, I want a portrait of Idra.