Not quite sure where this fits, but someone was commenting on habit building and how important that is (@leiafee?) . I think anything that encourages people to build habits, and get in little and often bits of practice is good.
Personally I have liked using Duolingo, in particular it is convenient to practice on the train when I would feel a bit odd doing SSIW lessons. I have a 94 day "streak" (which it measures) and I would hate to lose it, as I am aiming for 100 days. Also, as I started it and SSIW at about the same time and started them together I know how many days I have been learning and it gives me a sense of achievement.
Ideas within SSIW:
Something I have seen work well in other forums like this one are personal "diaries" - you just set up a section for journals, and anyone who wants to start one as their own record of their journey can do so. People still reply and chat within that thread, but it is that person's "space" on the forum to share whatever they want. Others can read them and get some inspiration when they see how others are improving. (Possibly allow any mix or combination of English and Welsh in these sections so improvement is visible? And records of the person speaking if sound and/or video can be incorporated?)
Profiles and learner stories in more of a blog format would also be good. I know whenever I start something new I search for people's stories for encouragement. These can be shared on social media for outreach. This is something I'd be happy to help with if it was a chosen idea as I have done a lot of this kind of thing for work.
Another idea would be for people to set regular challenges - eg "who's joining me in a 7 day sprint to do a lesson a day?" Or "sign up for July's challenge to listen to 100 hours of Radio Cymru" etc