Couple of thoughts, just spitballing without order of importance or anything.
These "old" forums are pretty much the last bastion of actually functional internet. Fast loading, no shitty active flash elements, slick and clean design - emphasis on slick and clean. I would prefer it to stay this way, without unnecessary implementation of reputations/upvotes and other reddit-style cancer. When I visit Juve and Milan forums it annoys me to shit that I'm bombarded by this cluster of information whereas I have to mentally separate all the overwhelming 800x600 signature gifs, animated avatars, points, karma pounts, upvotes etc to read a simple sentence of a post.
Why am I against upvotes and similar stuff like on Reddit you ask? The problem with these systems, functional visual aesthetic aside, is that they tend to lead to dishonesty and virtue signalling. People stop expressing themselves genuinely for fear of reaction from the community and start virtue signalling because that's what brings upvotes. That's why Reddit is an insufferable echo-chamber for the most part. Now you could say that "thanks" are not any different and I would typically agree. Ideally I would leave being able to thank someone but remove the counters and numbers. I genuinely believe that quality of discussion overall tends to improve when you remove the proverbial carrot on a stick from the equation, stops people from posting for the sake of posting instead of for the sake of providing a good argument to the discussion.
Facebook groups mostly consist of people who I wouldn't want to interact with much to begin with - these people value short replies and "fast food" type of discussion that is rarely proofread, has metric fuckton of emojis and other stuff and generally has poor etiquette. Likewise to giving one of those people instant admin rights or whatever here - that will only lead to separation and derision from both sides. In year 2019 they are not any different in terms of raw quality from people who still comment on youtube videos.
Smaller active community will always be better than a large community with sporadic posters. As far as growing bigger - I don't think you can manually induce this even if you wanted. Only way for that to happen is for club itself to actually achieve things by winning trophies. That's how you get new fans nowadays. Being a CL regular is a good first step luckily. I would rather reach out to the italian Inter forum and try to get some of those people here, that would likely yield better results.
If you look at other forums, Juve's isn't that much bigger than ours, if it is at all. All other Italian clubs have smaller forum communities than ours.