Somewhat Realistic:
New Deal - one with less restrictions on Iran. This deal gets negotiated by countries other than the US. The US continues sanctions, so Iran is only willing to agree to a deal that provides fewer economic benefits because the new deal imposes fewer restrictions.
No New Deal Version 1 - sanctions fall apart, with the US and maybe some other countries continuing sanctions, while China, Russia, and many others end sanctions. Iran takes detected and undetected steps towards a nuclear weapon but doesn't explode a device over the next 10-15 years. The US wisely decides against a military attack to "stop" progress towards a bomb, and Israel wisely decides it can't effectively stop Iranian nuclear bomb progress on its own.
No New Deal Version 2 - same as Version 1 except that US and possibly Israel bomb Iran. In Iran public support coalesces around hardline opposition to the Great Satan, reinvigorating the aggressive theocrats for another 20-30 years. Theocrats vow to rebuild the nuclear program and more support for terrorist groups, insurgencies, and Assad in Syria. In 3-5 years Iran explodes a nuclear device (or possibly, just build their nuclear weapon capability without finalizing it via a test).
No New Deal Version 3 - same as Version 2 except that a year after the first bombing, the US bombs again to destroy the rebuilt nuclear program and thereafter continues periodic bombing along with continuous attacks to degrade Iran's air defense capability. Other consequences of Version 3's long-term low-grade warfare are unpredictable but likely to be unpleasant.
Very Unlikely:
Renegotiated Deal Version 1 - the facesaver. After getting voted down in Congress, the US gets some very minor tweaks that provide political cover for enough Congressmembers to finally support it.
Renegotiated Deal Version 2 - The Unicorn! During a last-minute attempt at renegotiation after Congress votes the original deal down, somebody thinks of an approach that no one had thought of before that makes the deal somewhat better in restricting nuclear development while still being acceptable to Iran.
Same Deal Minus the US - Iran is so desperate to lift sanctions from other countries that it renegotiates a similar deal as before with a few facesaving changes, despite continuation of sanctions from the US.
One-off Military Attack - this is not the Military Unicorn believed in by the John Boltons of the world. Instead this is similar to the Somewhat Realistic, No New Deal Version 2 except that after a single military attack on its nuclear program, Iran decides to discontinue it, AND to get revenge by other means such as vastly expanded support for terrorists and insurgencies. Very hard to see why Iran would do this, given that the attacks just build public support for hardliners. Maybe they decide to put Iran's economic interest ahead of their own political interests. This is Very Unlikely.
Conclusion:
None of the realistic alternatives are better than the current deal. Only one of the very unrealistic options, The Unicorn Renegotiated Deal, is better, and it's both hard to imagine it being that much better while being much more likely to end up as one of the worse alternatives.
