Assuming I'm not misunderstanding your intent as much as I would like this idea I feel this would defeat the point of achievements. I don't mean to come off as condescending here, but they're meant to be a mark of just that - what you've achieved. If you see someone flying around on the Centurio Tiger mount, then you know that person has hunted their butt off to get that thing. Giving people different means to earn it sort of negates that work.
I know a lot of people were upset when they changed the veteran rewards system because of the amount of time it took to earn those items means very little now. I hope SE will add more items to the achievement shop, but as far as the ability to complete achievements I don't think it would be very fair.