The right collaboration with a current artist COULD work, but it is very risky and could cheapen U2 if it is poorly executed. I mean, look at the collaborations they've done in the past. Those with Green Day and Mary J. Blige I would argue were NOT good. Those they did in the 80s/90s (with people like BB King, Sinead O'Connor etc.) I would argue were good.
Are there any current artists who could collaborate well with U2? I think a strong female vocalist dueting with Bono could be very interesting so long as the music itself is a typical U2 sound (and by typical, I mean not cheap pop music). Lady Gaga has been mentioned here and it seems like some/most have a negative perception of her. While it is true she got famous as a sort of "shock" pop star, she has good vocal range and seems to be a talented musician in her own right. She also recently did a duets project with Tony Bennett that was surprisingly classy and good. Beyonce is another example of a strong female vocalist with talent (although her music is not my cup of tea).
Ed Sheeran was just mentioned (jokingly) and he, too, seems to be talented. I don't know that he could bring much to the table for U2, though. I mean, they're already working with Tedder and he's known for being one of the best pop song writers/producers in today's music world. I feel like anything Sheeran could bring to the table is something Tedder already has (and it did not work--see SoI).
But again, it all comes down to execution. If U2 stick to their style of music and decide to bring in a pop singer to sing on a track, why not? It could work out very well. But if they're going to just collaborate with a pop star and do a trashy pop song for the sake of getting another hit? That's going to just cheapen their reputation.