Where you're born is a bit much (Heaslip, Israel's finest rugby export) but I'd be up for strengthening the rules consideribaly
Can't remember who it was on here but they had a good points based threshold idea,
It was a while ago so I can't remember the details but it was something along the lines of:
Residence at different ages nets you different amounts of points, so if you move somewhere as a pro it takes twice as long as if you moved somewhere as a child.
Grandparents count towards eligibility but don't qualify you automatically, so you'd need a grand parent and be playing in that country for a couple of years to qualify etc.
It's pie in the sky but I'd scrap residency after you reach a certain age entirely - something like you can't become eligible for a different country after the age of 25 (so you could still join the final year of an academy at 21 and become eligible four years later)