Solution to Unicorn Chomper Issue
I believe there is a decent solution to the unicorn chomper issue that maintains the 313 GW1 achievement but allows people who don't have GW1 to get the unicorn chomper. That solution is to make it obtainable in GW2 but by a method that requires effort and dedication outside of just simply playing the game and levelling up.
The solution is NOT to just make it obtainable after reaching a certain rank in GW2. Why? Because everyone is doing this regardless of the unicorn chomper. It doesn't require any extra effort or dedication to achieve, like getting to rank 313 in GW1 does.
For example, it could cost a certain number of stars, say 3,313. This means those who want it have to put in all the effort to get these stars, while people who got to rank 313 in GW1 don't need to bother or spend time and effort doing this. If they did, they could spend those stars on xp or what have you.
I'm sure there are other ways of achieving this. But the general rule is this: make it obtainable in GW2 but by a method that requires extra effort, dedication and sacrifice outside of simply just playing the game and levelling up.