With the new DuoLingo Plus, it might be nice for people that want to give a little instead of subscribing to be able to buy fancy clothes and hats for Duo in the Duo Store (for money) instead. There's something about subscriptions that just bugs me. I would like to support Duo with a few bucks when I feel up to it, just not necessarily every month forever. Perhaps after some amount of time (a month or two or three depending on how fancy, maybe), the fancy hat or whatever evaporates so you need to buy a new one to show your support again. In the Clubs maybe you can see these clothing items so people know who is supporting and there is a little mild "peer pressure" to do it.
BTW, the "right" way to introduce a feature like this is to validate it with A/B testing. It's a bit of work but a no-brainer. You create a test version of the app (assuming there's enough support for the idea to even do that) that has "duo duds" and see if it makes more money than the current one. Some people see application A (current version) and some people see application B (duo duds). Run it for a month and see which idea wins.
You can unsubscribe to Duolingo plus after any amount of time, you do not have to do it forever. As for the peer pressure thing i believe it would go against Duolingos idea of free and equal education to all. If the community could see who was and was not donating money to Duolingo then people would be treated differently and this would suck for anyone who cannot afford to pay which is a lot of people so it would be like punishing poor people for being poor while giving better treatment to those who can afford it which is seen enough in real life and should not seep into an education site whos intention was to avoid this kind of stuff