Board Thread:Naruto Discussions/@comment-31437001-20170322074923/@comment-2006510-20170322093329

Expansion Jutsu isn't two natures; it's merely Yang-Release, just as Shadow Possession Jutsu is merely Yin-Release

As for how you make a kekkei genkai, your guess on a reliable method of creation, seeing as they're diversely different in that respect. Some involve the mixing of two-or-more elemental natures, but not all, such as the Byakugan/Sharingan/Rinnegan. The base definition is that it's some special/unique bloodline ability - whether it be mixing elemental natures (numerous examples) or the ability to create bones (Kimimaro and/or Kaguya herself), etc - so it's rather broad