What would you recommend?
What kind of trees do you like?
I'm a little north of you, and have had good experiences with seedlings from flowering cherries, black cherries (wouldn't recommend them, though, they're a pain in the ass as bonsai), redbuds (they flower prettily, but they have ginormous leaves), red maples (not good bonsai, but probably the most common tree around here), Norway maples (probably worse than the reds), and just about every apple seed I've stuck into substrate. If I can keep the seedlings alive and happy in spite of my care and stubborn insistence on using wholly organic potting media, you can keep older trees alive and happy. I've heard good things about Amur maples, but haven't tried them yet, and tridents are just a little less cold-hardy. I can't speak for any conifers; I don't think I've had one that's lasted more than a year. Clearly, I'm doing something wrong with them.
Note that none of those are tropicals; they'll all need to stay outside for the winter. If you're wanting something to stay indoors, you'll need a tropical and to set up sufficient light and humidity. Humidity trays and misting, despite what you might read, are
not sufficient. Get a terrarium or an aquarium for this.
Schefflera arborica isn't well-regarded as bonsai, but it's damn hard to kill and can get to look nicely tropical, and I'm pretty sure I'd need a priest and a silver sword to kill off my
Ficus benjamina. I've been moving it around every month or so, and I think the only time it dropped leaves was when I made the mistake of trying to bring it into the office (it, apparently, likes work even less than I do).
Portulacaria afra isn't properly a tree, it's a succulent, but I'm rather fond of it and the fact that the leaves taste like Granny Smith apples - you can taste-test them to see how they're doing (if they're not sweet enough, it needs more light) - appeals to me. Just call it a bonsai of a jade and watch the purists cringe.
All three of those are much happier outdoors than in, mind; they're just resilient enough to tolerate human habitat. If you stick them outside for the summer they'll grow much more.
