What about Bulbine Natalensis? You'd likely want to cycle it, but I was taking it with Piperine and the results seemed pretty notable.
I don't know what the general opinion around here is on the stuff, but I've had positive experiences with it.
Aside from that, you might look into Fadogia.
Outside of herbs and possibly going outside of what you call natural: DIM (a broccoli-family extract) is a estrogen regulator that could increase production of Testosterone - or, at the very least, increase the Testosterone:Estrogen ratio favorably - and D-Aspartic Acid can be used to increase the amount of FREE testosterone (not the production, as everything prior mentioned will / may do.)
Keeping your body fat low will help with keeping estrogen down, as well. If you want to go natural with the absolute fewest possible side effects or need of cycling, then getting regular exercise, good sleep, and keeping body fat low will do loads for that.
If you want to take stuff to help it along, then you're on the right track with ZMA and D3. Maybe get some omega-3s in there and read up a bit on DIM / Bulbine / Fadogia. I've yet to try Fadogia, but the studies are out there and seem very promising. Bulbine has potential liver risks tied to it, so that may be worth skipping. DIM and DAA are the gentlest that I've used that have unquestionably effective applications for T that could be considered natural (with somewhat generous applications of the term "natural").