From the wiki:
Packmasters are specially-trained Skaven who are experts at goading their charges – ferocious, half-mad creatures who can turn and attack with no warning. For this reason, Packmasters are themselves cagey and fierce warriors or, if they are not, they quickly end up as another meal for their merciless packs.[
I think it makes no sense that Packmasters are normal clanrats when in the lore they are described as warriors.
Gameplay wise they blend too well into the horde especially since there are a lot of elevation changes in the newer maps.
To improve the packmaster, Make them black rat sized and give them increased health to compensate if needed. This is to improve visual clarity.