So I'm beating this subject to death again, only this time I will offer a solution. Adding character changes as they level up can already be done as can be seen by the Magus disk and the White Lions pet so we know this is a possibility already to put in but the question remains, that if Mythic decided to put it back in, how could they? This is where I attempt to answer that.
First of all, lets give Mythic some money out of it. There will be a WAR booster pack out which would add to all classes and races a way to customise your appearance further depending on your renown rank, this is achieved through the use of special 'dyes'. If you have the collectors edition you know about the head dyes which give you the special faces, this would act the same way, allowing you to apply a new change to the character.
For Black Orcs I would propose the following.
Upon achieving renown rank 20 you get a mail in the post from a Sneaky Git letting you know that there is an old warboss in a zone that is ready to get a good kicking and this would be a great chance to push yourself up the ranks in Grumlocks Waaaagh! So off you trot to the location, find the Orc and give him a hiding he wont forget, seeing that you're a much tougher Orc than he is, he gives you some fungus beer and a leg of ham and slips off into the distance admitting that you're much bigger than he is. This beer can be used in the special dye slot to increase the size of the Black Orc by 1% It's not a lot but still, you're not yet a tough Black Orc like Grumlock. Also the leg of ham, if used in the special dye slot will darken your skin a bit.
Once you get to RR40 another mail appears again from the Sneaky Git. This leads you to a quest to complete involving blowing up stunties, much hilarity and then defeating another bigger Orc in a headbutting contest. Upon completion of this you receive another pair of dye items to use which increases the size of the Black Orc to 3% above base size and the other will darken your skin further, also you receive quite a large trophy to proudly wear showing you're bigger than other Orcs.
This process repeats itself at RR60. A quest which sends you off to take some Elf ears and track down an Orc which has betrayed Grumlock in exchange for his own pile of shinies from the High Elfs (which in reality is just a load of shiny stones). Once this is done, you will again receive the two dye items which once used increases the size of the Black Orc to 5% of the base size and darkens the gnarly green hide of your Black Orc even more. Along side this though you are granted a Shield, this has no stats and is bound to player, in fact it's a unique skin for the appearance system, it turns the shield into a High Elf shaped/design but with Orc glyphs and no doubt some extra spiky bits attached to it. Again, this shows off you have achieved RR60 if you want to use the appearance.
Guess what happens at RR80? The sneaky git reveals himself to be Gazbag who has been watching you. Having defeated an old Warboss in the orc lands, a warboss in the stuntie lands and a warboss in the elf lands it's time to head to the Empire and lay down Grumlocks law on a warboss which has had a siege set on Praag for months now without progressing and in fact is getting fatter, lazier and his boyz are getting more and more worthless as each day progresses. So a quest is set forth to get them sorted for Grumlock until you draw the ire of the big, fat warboss. Again, once you defeat him you feel the power of the Waaagh and as all Orcs do when they defeat a bigger Orc they themselves become bigger. The dyes you receive this time increases your size to 7% of the base, make you even darker and in addition you retrieve a choppa which is in fact the Runefang which was lost to a great Waaaagh many centuries ago. Again, this is only appearance, and not effect.
Finally, when all appears to be settled down, Grumlock calls upon you at RR100. The Skaven have used some evil ratty magic to enslave one of Grumlocks trusted Black Orc bosses. You have to go down there, break the enchantment (and a few skaven skulls), upon doing this the boss wakes up and realising that the only way to redeem himself to Grumlock would be to give you a solid beating takes you on at a fight. Once/if/when you defeat the boss (it doesn't have to be easy, just a challenge for a solo player) the now beaten boss, hands over a chunk of warpstone, a cooked rat and a massive, black warpstone enchanted two handed great choppa. The warpstone will increase the size of the Black Orc to 10% over the base, the cooked rat will darken the skin further and the great choppa is a unique appearance for great weapons.
And that's that! It doesn't need to go further, it can be expanded to other races and classes (dwarves get longer beards and more gnarled looks, Bright Wizards may gain more burn marks or flames erupt around them (oooh particle effects) etc etc). You also don't have to use each item, it's simply there if you want to you might like your skin the current colour so there's no need to change it, your size might not appeal to you and you may like the appearance of other items better. Whatever your choice though it's that. A choice, you're not forced to buy it as it offers no in game advantage and even if bought if you dont like the look of being bigger and darker, well heck, you don't need to use it!
I'd pay for it for sure.