Correct Schema.org itemtype for the related content

I am defining the itemtype for reviews as follows:

<div itemscope itemtype="http://schema.org/Article">
    <!-- Review content -->
    <aside itemprop="" itemscope itemtype="">
        Related content goes here
    </aside>
</div>

Which microdata should I use for the related content?

You are using schema.org’s Article, so you’ll have to use properties defined for this type (or proprietary properties in the form of absolute URIs).

Article doesn’t define a property for general relations, but there are properties for special relations:

• about, if it’s the subject matter of the Article
• citation if it’s a citation or reference of the Article
• comment, if it’s a comment about the Article
• mentions, if it’s a reference from the Article
• review, if it’s a review of the Article

The type of the related content, well, depends on the related content. The choice may also restrict the use of the mentioned properties, as they typically expect a certain (general) type.