I think that you should make some dwarfs rarer and have better stats. Then you buy them in the shop and you have to get to higher difficulties to unlock better dwarves. Unless there are rare dwarves and I just haven't found them yet.
Viewing post in Dwarves: Glory, Death and Loot comments
Dwarves actually have randomized stats, so it is possible to find very different ones. It is just not shown visually (maybe I should reflect that somehow). Most notably is probably the speed stat which can range from base 5 to base 15. You can hover over the dwarf before buying him to see the stats.
In the future I plan to implement permanent upgrade possibilities for the dwarves.