Djokovic will win at least 1 more slam before he retires but anything beyond that is a bit of a crap shoot. He could win 5 more or he could win just the 1. It really depends on whether the NexGen step up and push the old guys off the table or not. It really is such a free-for-all at the moment. Roger, Rafa, Nole Murray and Wawrinka should have 5-7 more slams to win between the 5 of them. It's just not clear how that 5-7 gets divied up. Guys like Berdych, Tsonga, Ferrer are already in the sunset of their careers. Can players like Nishikori, Dimitrov, Delpo, Raonic and Cilic make a move? Collectively they may snag a couple of Slams, but I think they will have a pretty narrow window in which to do so.
With the NexGen, the good news is that Zverev is hugely ambitious, Thiem is super dedicated and I feel that the more success these two achieve, the more Kyrgios will get serious about being an ATP pro.
But to answer the OP, I don't think Agassi will make much of a difference on Djokovic to be honest. Their styles are pretty different.