Let's assume that there is a cavity with a couple of mirrors and gain media between which possesses luminescence under some external excitation/pumping. Let the absolute quantum yield of the gain media be quite high - 50%, luminescence decay is approximately 100 µs and full width at half maximum is about 100 nm with maximum at 500 nm (something like blue-green light). All these parameters are quite real. So with such parameters what do we need to get lasing? And second question, what would the output spectrum look like?
Relevant link - not exact duplicate . It discusses the need for population inversion, and how to achieve it. A system with fewer than three (four if you want to limit power loss) energy levels is unlikely to be able to sustain population inversion, so while you could get "stimulated emission" it's unlikely that the output of such a medium would be coherent (there would be too many opportunities for spontaneous emission that destroy the coherence).