Translation:I want to tell you something.
I assume that you may only use 东西 when reffering to tangible things.
If you cannot touch it, use 事情.
So sometimes "想“ is translated as "would like" and sometimes it's translated as "want".. come on, Duolingo! Consistency!
Want and would like are anonymous in English. It is a question of style and both should be accepted.
Is "I want to tell you one thing" correct? I would have thought so, but Duo disagrees with me XD
They're not very loose with some translations, 'thing' would probably be considered too colloquial for what they're using, but in my mind totally acceptable especially with the counter prior.
No I think that "wish" (when used to mean "want") is fine. It is one of the translations of 想 in my dictionary. I think Duo just hasn't included it because "to wish" also has other meanings such as "hope for" or "long for". If you reported it, it would probably be included.