Well, he already did purposefully introduce something in the new timeline. He gave Scotty some "help" figuring out his Transwarp Transport equation thingamajig, decades before Scotty would have otherwise worked it out on his own. He should do him another favor, and warn him to never step foot on the Jenolan. XD
Overall, since he's stuck in this "alternate" timeline, to which the Temporal Prime Directive likely doesn't apply much to, he'd probably see fit to warn the Federation about a few of the "big" things that might threaten it or its present and future allies. Like V'Ger, the Whale Probe, the destruction of Praxis, the Borg, the freakin' Nexus, etc... It'd be hard to predict how things would change, but the Federation and humanity might end up better off for it.
As for introducing more advanced technology... Well, Spock didn't exactly bring much of that back WITH him aside from the Jellyfish, which was destroyed. But Starfleet already got itself a nice boost in the wake of the Kelvin incident. Didn't even really need Spock for that one.