Prioritise language skills
Normally we have all four different skills in our lessons, but I thought it would be helpful if we were able to choose which skills we want to practice (e.g. like in Lumosity). You could prioritise which of the four you want to spend the most time on: Speaking, Writing, Reading, or Listening. All the exercises for each skill already exist, its just that you will be able to choose what you want to practise - what is the most important thing for you. E.g. I have a writing exam coming up and don't want to practice speak. or listen. skills - only write. and read.
Believe it or not, learning how to pronounce a word helps you remember how to spell it and vice versa. Learning to spell a word helps you learn to recognise the sounds of the letters which makes it easier to recognise or work out what a word is when you hear it etc. It fills in your brain's 'file' for that subject and provides more mental connections which allows you to retrieve any of the data in that 'file'.
Practicing 1 of those skills will improve the other 3 as well.
I understand what you are saying and think it's right. But don't you agree that one of the exercises is bit toooo easy? I mean the one where they give you a choice out of three sentences and make them very easy and silly e.g Find the translation of 'They drink coffee' 1. Ils boivent du cafe 2. Ils boivent du chat 3. Ils boivent du robe - I personally find this exercise useless and quite annoying, and think it would be cool if I could manage my exercises and simply switch this one off my list. Hope this makes sense :)
Yes definitely, the ones which include pictures are ludicrously easy and I really don't think they help much. However perhaps other learners find them useful? I do not really know much about the learning practices and preferences of Duo 'students'.