Gabler
Masterpiece
I think sometimes its good to be constrained a little though, especially in contest situations, as it takes more of a forward thinking eye. What sort of rock would suit X genus the most, what style of tree, what composition etc. Forcing a beginner or less experienced person, such as myself, to narrow down those selective options can make for more of a learning experience than throwing mud a wall to see what sticks.
I don’t think that a specific species would be right necessarily either. I like @Gabler ‘s suggestion of a genus instead. For example, most places in the world have Acer, Malus and Prunus native or imported pretty readily and they’re also reasonably quick to develop (though not necessarily to flower).
To clarify, my knee-jerk reaction was to think, “Ew. Gross. Species restrictions? No way!” But then I thought about it a bit to figure out why someone would want to limit the species selection, and I must confess I recognize there are some advantages. I suggested a genus restriction in lieu of a species restriction as a sort of compromise. That idea backfired, as now there are three camps instead of two. I’m still in favor of a genus or family restriction as a middle ground amongst the camps.