I like just saw the snapshot and the panda's are AMAZING and pillagers and their beasts and everything!!
I mean, even if you did, it would be very cumbersome as adding in custom mobs that aren't made from existing mobs is impossible plugin wise, but possible with mods. This would be a new feature when minecraft's new API is released tho. Which would be nice. Also, 1.13 really changed how mobs, syntaxing and blocks IDS work (internally) so it would be very hard to just kind of "remodel" it in an older versionTbh at this point it might even be easier to take the already exisitng coding for panda's and water mechanics and just make a plugin that transfers them over to 1.12. (While ofc this also is not an as easy task as it looks obv). I heard some dev's talk about it in a discord group I'm in. But anyway, there are already enough improvements to be made in 1.12 for this and other servers. That's why a server like Hypixel still has 1.8 parsts of the server and 1.13 parts.