It is a more sophisticated way to say something, but it is not a mistake!

You would need to add a conjunction after deem. "I deem that this idea could be successful". However, "deem" is formal and not commonly used in everyday speech. "Consider" and "deem" mean slightly different things. "Deem" comes from the old English word "dēman" which means to act as a judge so it is more conclusive. "Consider" is a little bit softer and is used either when you are still thinking through something or want to make it obvious that you are stating your opinion. A more direct alternative might be "I have decided that this idea could be successful." I hope that helps!

'I consider...' sounds more natural in English. Or you could say 'I believe this idea could be successful.'

I deem that this idea could be successful. OR I deem this idea to be potentially successful.