I think my only concern about Fonseca--aside from the specter of injury that is a concern for all young players, until they establish themselves--is that there's a quality of slim margins with his play. He can do so much, but everything seems to require slim margins: making those winners, getting his angles. Fonseca shows good defense, but his game is primarily offensive. He's going to have spells where he might be "Fonshanka"...and/or get flustered by just missing his shots.
Right now, he almost reminds me a bit of "Federer 2.0"...the later version of Federer that had lost a bit on defense, and thus couldn't fall back on it when his shots weren't there. We might see runs like Roger's 2017 when he was hitting so well and nigh unbeatable, but also 2013, when he was shanking all over the place and couldn't find a rhythm. Just speculating, though. He's got a lot of room to grow, still.