I completely support this decision. I was somewhat sick of the old world, if anything. And updating to 1.9 would give idiots like me a fair shot with PvP (until someone gets 1,000 axes or unarmed, which will take less than 48 hours if we're being honest).
I believe I remember someone saying commandblocks are disabled for the server, but maybe you can enable them on this new one (yes, I know how servers and worlds work - you would have to enable them on the entire server, but this is just a suggestion). This would give the admins room to experiment with commandblocks a little, instead of relying on plug-ins. Personally, I dislike modding my Minecraft because I'm somewhat experienced with commandblocks, so I've always thought, "everything you can do in a mod, you can do with commandblocks." This holds true a pretty good majority of the time - it just takes a while to actually get used to commandblocks, of course.
By now, I think the SkyBlock world is a bit more dead than it was, maybe, a year ago. Perhaps it's time to restart on this as well? I'm sure there is another SkyBlock plug-in that is a bit more expansive than the one we have now. Also, this is the only server I've played on that gave you a limit for your island. I'm not saying I don't like, and I'm not saying it's wrong ("what can you do in this amount of space" kind of theme). However, a lot of new players want to play SkyBlock, and having an inferior SkyBlock plug-in may be the death of the world on our server. It does tend to get a little boring if it isn't updated. Further, I would like to add that my McMMO transferred from Pern to SkyBlock. If we're resetting our McMMO, will this change it in all servers?
(The reason why I'm talking about different worlds is because I'm seeing this update as an opportunity to change the server in general. If you're going to update part of it, you may as well update the server in general.)
As for changes in minigames, I think Mob Arena could use one. To be honest, I feel reluctant about this opinion; I don't know yet whether I support it or not, but I do want to bring it to your attention in case anyone else does. Some Mob Arena-like plug-ins have the ability to upgrade your classes, if not make a custom one. They also have "abilities," which are just "magic spells" - something that can be used to attack, support, used for movement, etc., and has a cooldown. The only problem with this is that it makes Mob Arena a lot more different than it was before. Honestly, I'm fine with any change this server has to offer, but having "upgradeable" classes could make Mob Arena a bit less competitive (To put it in perspective, you said something about bragging rights and a stats board. How can someone brag about killing another player if it was 1,000 axes vs. leather armor?). As for me, I'm fine with upgradeable classes, as long as the upgrades aren't too overpowered (maybe +1 or 2% of a stat for every upgrade).
Obviously, I'm sure you will fix the minigames that are currently broken during the change, or soon after.
Also, a custom mobs plug-in could be fun. Instead of the boring mobs that always knock me into my 32x80x64 hole and kill me, there could be some harder mobs that occasionally spawn. So maybe a skeleton with an overpowered bow, or a zombie that gives you a Wither effect. I've also seen "bosses" that spawn every 15 minutes to an hour, along with a chat message that says "A boss has spawned near X Y Z coordinates!" These bosses could be vanilla, such as a skeleton with lots of health and a fully automatic bow, or a bunch of Ghasts riding a zombie with lots of health. Fighting these bosses would give you a ton of XP, money, or some other cool rewards. Or maybe even the most overpowered item in the game, a Mending book. You said something about adding some "adventure" aspects. I don't know exactly what that means, but I feel like these custom mobs could support that idea.
^^^ If anything, I think the custom mobs is the most reasonable suggestion I even had, and that deserves my full support. I'll actually be a bit sad if I don't get something of a challenge from the hostile mobs - I don't think we die enough on this server.
Sorry for any bad grammar. It's pretty early where I am and I was playing the IW beta all night (I'm disappointed with it, but I play mostly zombies anyway). In any case, whether you're Fed or not, please at least skim through here and give me some other opinions.