Well, SP could have some gameplay bonuses for achievements. For example, there's a set of achievements for giving stuff to other players. Completing all of them can unlock "diplomat" status/title, which then gives you a 10%/20% longer mission duration in game. Completing the Go Big Or Go Home + other capitals related achievements can reduce the cost of capitals by 5%, or increase their hit points by 5%. Stuff like that. Every time you start a new SP game, you select one "title" to take, so you don't get all the bonuses stacked on.
Of course it's not going to happen, but if it were..
I like this idea! That is the reason why i bother with Mass Effect achievements, they actually effect something.
Raised Him From a Pup Myself: Get a capital ship to lvl. 10 through experience.
This already exists: "Capital Ship Captain" or something..
And for accomplishing all achievements "Gotta catch 'em all", of course (No, i'm not a Pokemon fan )
Or maybe something related to the games name "The Sinless One" or "The Mortal Sinner" whichever is more approriate.