One of the coolest periods for Twitter was when the API was robust and smartphone app design was taking off with small developers, and so you had all this cool experimentation in apps and UIs, that then made it back into the main platform itself. Then Twitter kneecapped the API and that all ended.

Mastodon offers the possibility of that fun experimentation again, and if it happens, it'll mean lots of fun app design, but also rapid improvements. Fingers crossed.