For new sign-ups, expire the membership level on Dec 31st, the year after they signed up.

pmpro_next_year_expiration.php

<?php
/*
Plugin Name: PMPro Expire New Memberships Next Year
Plugin URI: http://www.paidmembershipspro.com/wp/pmpro-customizations/
Description: Customization to expire new membership sign-ups on Jan 1 the year after, the year after this one.
Version: .1
Author: Thomas Sjolshagen @ Stranger Studios <thomas@eighty20results.com>
Author URI: https://eighty20results.com/thomas-sjolshagen/
*/
function pmpro_next_year_expiration($level)
{
	//ignore renewals (they won't be changed from membership level definition)
	if (pmpro_isMember(null, true))
		return $level;

	//add to this array (level ID) => (expiration date in yyyy-mm-dd)
	$nextyear = intval(date("Y")) + 2;
	$custom_expiration = $nextyear . "-01-01";

	//needed below
	$todays_date = current_time('timestamp');

	//check the passed level against your array
	//how many days until expiration?
	$time_left = strtotime($custom_expiration) - $todays_date;
	if($time_left > 0)
	{
		$days_left = ceil($time_left/(60*60*24));

		//update number and period
		$level->expiration_number = $days_left;
		$level->expiration_period = "Day";

		return $level;	//stop
	}
	else
	{
		//expiration already here, don't let people sign up
		$level = NULL;

		return $level; //stop
	}

	return $level;	//no change
}
add_filter("pmpro_checkout_level", "pmpro_next_year_expiration");

function pmpro_isMember( $user_id = null, $inactive = false )
{
	global $current_user;
	global $wpdb;

	if (empty($current_user->ID))
		return false;

	if (empty($user_id))
		$user_id = $current_user->ID;

	if ( false === $inactive )
	{
		$sql = $wpdb->prepare(
			"SELECT COUNT(id) AS is_member
				FROM {$wpdb->pmpro_memberships_users}
				WHERE user_id = %d",
			$user_id
		);
	}
	else {
		$sql = $wpdb->prepare(
			"SELECT COUNT(id) as is_member
				FROM {$wpdb->pmpro_memberships_users}
				WHERE user_id = %d AND status = 'active'",
			$user_id
		);
	}

	$is_member = $wpdb->get_var($sql);

	return (!empty($is_member) ? true : false);
}