How many people that buy packages end up making something worthwhile and shippable? I get that you’ve got interest in this but I think you are overestimating the UE4 fighting community.
People buy packages mostly because they can’t produce the needed asset themselves be it code, art, sound, or anything else, and to a lesser extent because they can’t be bothered with making something and they go for a already made solution. You don’t need to reinvent the wheel, that’s why we’re using already made engines and not writing our own, it does most of the stuff we need already. But I dare say gameplay is different animal, it is specific to each game even though it can be mostly similar between them. Good programmer will make the gameplay system by himself (or his team) and won’t buy an “Ultimate Your Genre Of Choice Maker” that is made god knows how and will most likely be a hindrance in the long run (if the system isn’t 100% on board with your gameplay needs you gonna do rewriting).
There is not that much people making fighting games, most of the guys/gals buying “fighting game engines” are probably not experienced programmers, small fraction of those will have networking in mind from the start of development, even less will get to the point of having a full game that can be played vs bots or on one system with 2 players. Amount of people that know that they have a need for proper rollback networking is pretty small and they will need good networking programmers for that job (as you said even AAA games have ■■■■ time with this). Good network programmers will either work at a game dev studio that can pay them, or they will make good money somewhere else.
To have a community made plugin, you need a critical mass of people that know they need different networking for fighting games and a good networking programmer that will pull it off.
Good luck, have fun.