For most of the last decade during tennis' "golden age" on the men's tour, the sport has mostly steered clear of major controversies. The Big Four clearly reigned supreme, and everyone else seemed to know their place. There may have been some minor dustups between members of the Big Four (notably Federer and Djokovic), but they've largely gotten along given how often they run up against each other. They all seemed to realize they're more marketable if they "keep it classy", and have taken steps to maintain pristine images.
It looked like the next generation was going to follow in the Big Four's footsteps in terms of their images. Kei Nishikori, Milos Raonic, Juan Martin Del Potro and Grigor Dimitrov all seem to be liked by other players and fans, and minus a few harsh words by Serena aimed at Dimitrov there hasn't been much controversy following this group. However, they've also failed to take the next step to consistently challenge the Big Four on the court (or are at least taking their sweet time in doing so).
Just when the ATP Tour thought it was safe and that juniors would learn from the Big Four, along came the even younger and more talented group of players such as Nick Kyrgios, Thanasi Kokkinakis, Borna Coric, Alexander Zverev and Andrey Rublev. This group has gotten a lot of attention following Kyrgios' recent controversy, and people seem to be realizing what the tour could be in for as the Big Four begins to wind down. (Though at this rate it seems like Federer will still be firmly entrenched in the top ten when he's 47). All of the above players are extremely talented and should be some of the tour's best over the next decade, but are widely considered to be varying degrees of temperamental or controversial.
Everyone has their opinion about whether the tour is better or worse with some controversial players at the top, and it's hard to reach a definitive answer. Some tennis purists will hate it, others will enjoy the added intrigue, and the rest will find some controversy entertaining so long as it doesn't go past a certain point. Regardless of one's opinion on the issue, it's looking like it won't be long until we once again find out what it's like to have controversial figures ruling the sport.
There's a chance they'll tone it down as they get older and realize they're losing potential endorsements. Djokovic and Murray were considered controversial to some extent when they were newer on tour, but now both are incredibly well respected. However, at this point it's hard to see all (or even most) of this younger group making such a transition. Assuming they don't, tennis fans may soon feel like they're stepping back into the McEnroe and Connors years of the 70s and 80s, as opposed to following the "golden age" of the Big Four.