I was quite interested in the Ally dragon idea, and as quite a lot of allies in Skyrim, there is normally quests for them. So with that in mind, after you have made them allies, you could build your relations with them, learning some of their shouts possibly, the same way the greybeards teach, possibily also allowing the 'Real' best forge in skyrim as Companions do, the Skyforge is great and all, but Uniquely being able to make Draugr armour isn't really that good of a benefit, if it was required for anything past dwarven, or elven, then I'd be more interested in taking the time to get over there. You could use this forge as the only place to make Dragon Armour, not dragonbone, or scale, but rather made from unique metals at incredibly high tempreture (Dragonfire powered) to make armour that is equivilant to Deadric or Ebony with 100 smithing. This could also introduce not only raiding dragon lairs for loot, but maybe to mine precious metals that they also hide?. A fire dragon would probably be quite a good choice, however not a caster. I was thinking of Fire and it's more of an aggressive, personal kind of thing, people fight close up when 'fiery'. So instead of it caster, I would think close ranged shouting / breath and melee attacks. Can maybe make it so it can 'enchant' it's claws to be able to set you alight much like an enchanted sword, which would give it a more ferocious aspect to it, especially since most people use bows and spells to take dragons on until they land. He lands often, and he goes for the kill straight away. But then again, thats when he has an enemy, if he realises that you are not already an enemy, he may be willing to trust. I liked the Farkas reference as I feel he has the right attitude to people, however maybe alter it to be more like Aela (Male version) because she can trust, but is smart enough to know why to trust etc.