Proposed syntax for SilverStripe template includes with content as a parameter like Sass mixin content.

AboutPage.ss

<!------------------------ AboutPage layout -->

<header>
	<!-- Header stuff -->
<header>
<% include HeroBanner %>
	{$HeroBannerContent}
<% end_include %>
{$Content}
<footer>
	<!-- Footer stuff -->
<footer>

HeroBanner.ss

<!------------------------ HeroBanner include -->

<div class="hero-banner">
	<div class="wrapper">
		<% if $ParameterContent %>
			{$ParameterContent}
		<% else %>
			<h1>{$Title}</h1>
		<% end_if %>
	</div>
</div>

Page.ss

<!------------------------ Page layout -->

<header>
	<!-- Header stuff -->
<header>
<% include HeroBanner %>
{$Content}
<footer>
	<!-- Footer stuff -->
<footer>