will-o'-the-wisp
1. A unrealistic, unattainable, or delusional hope or goal. The mobile phone market has proven a will-o'-the-wisp for the giant tech company, despite the huge amount of resources they've poured into staking their claim in it. As we've seen time and time again, attempting to change or repeal the amendment will forever be a will-o'-the-wisp.
2. A person or thing that is or seems to be fleeting, intangible, or illusory. The president's stance on the issue has been something of a will-o'-the wisp, seeming to change or disappear entirely as the political climate dictates. The linebacker has been a will-o'-the-wisp all evening, appearing seemingly out of nowhere to deliver bone-rattling tackles.