I'm mostly looking forward to the continued evolution of our style of big baby diapers. I think there is still room to grow with features and aesthetics. I also would love to begin seeing some more sustainable brands of quality AB-style diapers out there, made with more environmentally friendly materials, in addition to perhaps some specialty products for specific (niche, or not so niche) interests. Say, a disposable adult "potty training" lineup for ageplay.
I'm not sure if the community size-wise will grow or not, but it doesn't super matter to me. What happens happens. There are good reasons for the numbers to go both ways.
As far as visibility and mainstreaming, I'm generally not for it when it comes to "weird" subcultures and interests...parts of the community tend to get gobbled up by major corporate interests looking to make a buck, which is a huge turnoff. A creative economy in subculture and dedicated insider industry is awesome, aggressive external corporate hegemony is not. If furry ever goes that way, for example, I'd be out super fast. However, I'd say relative mainstreaming would be a boon for this particular community, given that it's rather centered on consumption of goods (namely, diapers). I have fever dreams of Kimberly-Clark spitting out pallet after pallet of adult-sized Huggies, and the US congress telling the FDA to knock it off with the designation of adult-sized diapers as medical devices. So I think it could be good even if we only got a quarter of the way there. That being said, I'm not holding my breath. I think we'll be "weird" for a long time.