No doubt considered a stupid idea by many, but I’d love to see engineers be able to make hunter pets.
Obviously it’d work differently to making mounts of battle/companion pets, but I’d still love to see more mechanicals in game.
I do understand the programming difficulty involved, but that doesn’t mean it couldn’t be done. 
Personally, I’d prefer if Blizzard opened up the current mechanicals roster more. There are quite a few mechanicals in Undermine that we could tame, but Blizzard has yet to flip that switch, such as the small mechasaurs and the weird walking boxes you see in the Undermined Raid. 
Let us tame Flarendo, you cowards.
There is one pet you can assemble, the colored mechanical scorpions in Siege of Orgrimmar
Yes, I have those and got all the paint pots as well. It’s a pity you can’t “assemble” the sabertrons into one bigger one too.