I think it's what you define as 'major': a conventional strike against US forces or assets? Very, very unlikely. A proxy strike against US assets? Much more likely.Yeah they'll be some sort of response but nothing 'major'. Remember they have just taken out their mastermind.
Not really sure Iran has any powerful allies as someone stated on here.
It might not come straight away or even in the next few weeks, but there alliances (less nations and more militias) will be used to some extent to retaliate.
I suspect it may not be military personnel who face the brunt of this: it'll be softer targets that the incite and outrage the public.