The answer to the OP’s question is also depending on what actually can count as a 'discovery'.
Would it only include species that the finder is able to recognize as ‘new’, based on morphology/coloration - or that a certain finding just evokes suspicion that then has to be subsequently confirmed by an expert.
In the latter case, my take on this topic would be: check (rare) plants for galls, mines, scale insects, psyllids, mildew and other tiny organisms.
Because IDing the plant would be easy and as a next step, search available sources whether some relationship is mentioned for the respective taxon.
Go one step further, collect the galls and mines and try to get some parasitoids out of them - the chances are even higher of discovering something new, but you very likely won’t be able to tell until an expert has look at them (might even include DNA barcoding).
I once collected a not so commonly found Drosophilid in a Botanical Garden. It was found on a fungus, but had itself a parasitic fungus growing on its head. In the literature, there was no mention of this genus being infected, I sent it to an expert - et voilà! A new species!