Five experts - relatively freely putting out their views. 

[Giora] Eiland also offers a counterintuitive idea: Introduction of a nuclear weapon by Iran could create communication channels between Jerusalem and Tehran that currently don’t exist. Citing the American-Soviet Cold War scenario and relations between two other nuclear foes, India and Pakistan, it is clear that some kind of an Israeli– Iranian dialogue will be needed in order to avoid a nuclear catastrophe. “We should also remember,” Eiland added, “that at the end of the day, Iran is a natural ally of Israel after the current regime falls.”

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According to [Ash] Jain, a nuclear Iran could provide Hezbollah and Hamas with a “nuclear umbrella” that will allow these groups to carry out attacks against Israel without fear of major retaliation. “In this context,” he added, “Iran could provide them with chemical and radiological weapons that will be directed at Israel.” He also argued that the chances of reaching a two-state solution between Israel and the Palestinians will decrease significantly once Iran turns nuclear, since extreme players such as Hamas will be bolstered while the moderates are sidelined.

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The worst outcome, however, will be felt in the United States. A nuclear Iran, [Shoshana] Bryen believes, will show the world that America “has no credibility,” since it did not live up to its promise not to allow Iran to become nuclear. “So what good is it to be a friend of the U.S.?,” Bryen asked.

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In either case, Iran’s goal is to assert its position as a major player in the region, one that the world should take seriously and with which it should consult. “Iran feels that it is entitled to that,” he said. “They have an image of themselves as natural leaders.”

[Gary] Sick, currently a senior research scholar at Columbia University’s Middle East Institute, does not see an imminent change in Iran’s regional behavior if the country gets closer to having a bomb. It already supports Hezbollah with all its force, and it has not shown much of an appetite for meddling in its neighboring Gulf countries, Sick said. “Their foreign policy has been not to open wars and conflicts,” he noted. But this cannot be interpreted as adopting a moderate policy toward Israel. “If Iran will be able to make life miserable for Israel, they’ll do it,” Sick said, “but this doesn’t mean they’ll go to war.
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