It's not a terrible thing to build, but whenever you make a product that's just imitating someone else (Twitter has 'tweets' but Mastodon has 'toots' because the logo is an elephant!) then it needs to be way better, not just a slight improvement. Longer messages are a good thing on Mastodon, though Facebook already does that. But what else does it offer that offsets the confusion of finding target instances, or conversely not being easily findable by people you don't want to converse with?
99.9% of users do not care about federation as a principle, it's just another level of technical gabble that they don't wish to be distracted by. Virtually every decentralized service struggles with this issue. Decentralization is primarily of interest to nerds, and for online services that requires you to be a bit of a computer and a bit of a politics nerd, shrinking your already small target pool.
Twitter's original win was that it was staggeringly simple, just asking new users to post about 'what they're doing right now' and offering simple controls to reply, repeat, or express approval. They realized that people felt more connected to a scrolling ticker of headline-style status updates than a newspaper.