I feel adding legendary keys to the voting shop would be a good addition and might get some more people to vote, though I do think they should be expensive enough as to not devaluate them too much. If we're sticking to 5 sites/5 points a day, 100 points each sounds reasonable to me. Towny/grief prevention claim blocks would be nice to have as well. Also always happy to see new spawn eggs as I'm collecting them, especially the new ones (axolotl, goat, glowsquid) but perhaps also things like bee, vex, snow/iron golem or drowned.
I'd be against adding netherite and NPC tokens, since myself and others have spent a lot of time and money (including real money in some cases) getting/trying to get those items. Adding them to the voting shop would devaluate them far too much and basically screw over everyone who've bought them before. Even more so in the case of NPC tokens, which (that is how I see it at least) are meant to be one of the most exclusive items in the game. As a side note, I'd rather see netherite in /shop for 3000 when bought and 1000 when sold.
Besides the regular /fly coupon, I'd love to see an alpha /fly coupon. Could even become fairly popular since donators normally don't get alpha /fly either.
Monthly prizes are a little tricky, since most of the good candidates have been a voting reward in the past. Temporary boosts would be a good choice, e.g. alpha /fly, McMMO boost, etc. Besides that there's still some useful commands like /itemlore - I think that's an Essentials thing? - and *cough* /top *cough*. Maybe more Skyblock rewards, w/e requests or a token to give an item a colored name?
Edit: Other random ideas - colored item names and /nv