It has been a while since my manga phase, but I really liked Motomiya Hiroshi's
I'm still Alive, which I've always considered mature in the sense that only an adult could have written it. It's a story centered firmly around the idea that it is never too late to live your life as something better.
While it's not as tight or as deep,
A Beast Cries in the Wilderness has art that is just to die for, some of the best animal/human hybrids I've seen, with cool artist notes about some of the decisions behind the final designs.
I confess to never fully making my way through
Oyaji, but what I did read was as anti-Maia as you can get. Badass old man dying of cancer decides to reconnect with family through the power of love and violence.
Korean, and a bit more mainstream, but
Covertly, Gloriously may also be worth a read.
There's been a lot of talk about manga as a whole throughout the thread, and you implied you preferred stories with some substance behind them, so here's a few of the ones that I can remember.