Tango is improvised? People say that, but I wonder if it's the right word. You could say disco is improvised: it's not really a partner dance and you can make up your moves as you go along, wave your arms around, jump up and down, throw your head back and forth. In tango you are holding your partner close to you; there are limits.
Ricardo Vidort said you can make up moves in tango, but tango itself is a limited framework, which is part of the beauty of it. I think 'making up moves' is more like finding new connections between things you've done hundreds of times, and practised through dancing. In effect, you've practiced over and over again even the simplest things like taking a step forwards until, as Fred Astaire (who practised obsessively) said, 'your body tells you what to do', you no longer have to think about it or make a conscious effort, your body is free to dance, to be spontaneous. Astaire's dance, of course, at source wasn't in the least spontaneous – it became spontaneous with practise.
Tango might be spontaneous, rather than improvised. There's an element of freedom, but so much of what forms the dance is very far from being improvised. Part-improvised, perhaps, in the way musicians improvise music: their music arises from working on practising their instruments, practising improvisations over and over again, probably studying harmony and counterpoint, and listening closely to improvised music, too.