I don't see Norwegian offering anything to IAG as a brand.
I think it would quite a good fit actually. IAG is clearly looking to move into the "millennial" market with Level and new additions such as Basic fares on BA longhaul. Norwegian is perfectly placed for that.
On long-haul IAG can simply close Level and allow Norwegian to continue growing their point-to-point routes across Europe. BA could reduce their LH presence at LGW, probably just keeping the Caribbean network which fits well with BA Holidays. As a group, IAG would have LHR/MAD/DUB as their hubs focusing on business travel and connections, then Norwegian focusing more on point-to-point leisure. Each airline gets to focus on what it does best without having to try and compete with each other.
Short-haul is more of a problem. I guess Norwegian could take over some of BA routes at LGW but there is still going to be significant overlap. IAG would presumably prefer a single LCC. In terms of brand awareness, Norwegian is much better known outside of Spain and even within Spain the Vueling brand has taken a bit of a hit in recent years. There's a good argument for merging Vueling into Norwegian. TO effectively close down Vueling would be a big step for IAG though and it seems unlikely.