You have to realize how Trump thinks. Everything is about him. If a deal doesn't make him personally look good, it's a bad deal. The Paris climate accord might be an objectively good thing, but signing it would make Trump look bad because he made an election promise not to sign it. Hence, it's not "fair" to the U.S.. However, if Trump can somehow make it look like he personally renegotiated the accord to get the U.S. a better "deal", then he can sign it because it'll make him look good.

The problem with the Paris climate accord is that it's vague and lacks any kind of enforcement. Everybody's signing it because there really is no concrete obligation attached. While some signatories may have signed it in earnest, others likely signed it just to look good. They'll announce a plan to meet the goals of the accord and then leave implementing that plan up to their successors, or just forget about it if they happen to remain in office. Look good now, don't pay later.

Given the nature of the Paris accord, there really is no better deal for Trump to negotiate. There are no binding terms to haggle over. I suppose they could change something in the deal just so Trump can say he did it and sign a "good deal". That's probably the best outcome.