So, I've seen this on a few servers where the players can purchase different spawners at the market, and they would either buy a certain type of spawner or buy one and use /spawner to customize it.
If the price for spawners are high, then it would give players another thing to work towards, and also enhance peoples bases that aren't alredy built near mob spawners. Perhaps players could donate for certain types of spawners, and they would come in packages like the disguises. This is obviously not a massive addition, but I believe it would make smp more enjoyable!
Thanks