widget-proposal.php

<?php
class WidgetArea extends DataObject {
	static $belongs_many_many = array(
		'Pages' => 'Page'
	);
	static $many_many_extraFields = array(
		'Pages' => array(
			'SectionName' => 'Varchar'
		)
	);
}

class Page extends SiteTree {
	static $many_many = array(
		'WidgetAreas' => 'WidgetArea'
	);
}

class TextWidget extends Widget {
	static $db = array(
		'Content' => 'Text',
	);
}

class AdWidget extends Widget {
	static $db = array(
		'Content' => 'Text',
		'Height' => 'Int'
	);
}

class ImageWidget extends Widget {
	static $has_one = array(
		'Image' => 'Image',
	);
}

Page.ss
$Content
<% WidgetArea('sidebar') %>