Here's the deal with replacing Twitter: first realise it is (under a sane interpretation) a really badly designed product, because it doesn't know what it is. Let's say that roughly it comprises:
1. A way to hear about news from people or organisations you like (Elon, Yankees, WTO, Lady Gaga, whatever)
2. A way to microblog and share with your friends or (more usefully) with colleagues or interest communities.
3. A discussion medium where there is no hierarchy of competence, and everyone's opinion is given the same weight.
(1) and (2) are generally positive and useful things. In (2) you might get a flamewar in a gardening community or whatever, so you need moderators to prevent outbursts over how best to deal with greenfly, but generally these kinda things are pretty peaceful. See HN.
It is not at all clear that (3) is a good idea at all. Most people know nothing about most topics, so giving equal weight to their voice is a bad idea.
The reasons that (3) has come about, persisted, become central to Twitter, is that it's a very good way to engage people. Enragement is engagement. It sells advertising, but it doesn't do much for humanity or its user base.
I'd say using Twitter for (1), leaving (2) to better platforms, and realising we don't need (3) is a sensible solution. The only reason we don't have that is Jack knows full well that (3) makes money, and to hell with the terrible problems that causes.
tl;dr - before looking for alternatives, ask yourself if you need anything at all.