Not a bad idea. In fact, I support it. Test cricket is the heart of Cricket and with the introduction of Test championship, it is going to be far more interesting now as draws will be played less (Indians have notoriously high draw rate abt 45%). Also there will be variety of taste as all will play against all.
For 10 team ODI WCs, I think they should make it to 12 as there are already 9 test playing nations which leaves just 1 spot for associate members. To promote more of cricket among this members, it will be a good idea if they let a couple more of associates to play in WC. this will definitely boost their moral of team selected and promote healthy and fair competition among them. As far as the length of WC schedule is considered, they can restructure in many various ways.