"Heavy use" is such a vague term IMO. Screen on? WiFi or LTE/HSPA? Gaming?
Without resorting to third party batteries I can't even think of any phone that can consistently handle 18 hours of just music streaming over cellular data. Maybe a Droid Maxx.
		
		
	 
I think there's more we can do with today's phones. There was a discussion on Reddit regarding why Android phones use so much battery in surfing the web. A lot of fingerpointing was done at Project Butter, which reinforces what I believed before. The whole scaling of two cores to 1+ GHz may be nice to provide smoothness, but in many ways its overkill. It might be why if I surf on my iPhone on LTE, it will get warm, but surfing on my Nexus 5 on LTE? It will get much hotter. Now of course we can cite the aluminum frame or whatever of the iPhone 5, but I'd say even with the old iPhone 4 or 4S design, it still didn't heat up as much.
Furthermore, there's a lot more software tweaking that can be done to minimize idle battery use. My proposal is to have further syncing options. For example, does Google+, Gmail, etc really need to sync in order to receive email? They hold the phone awake forever. There should be a battery saving option that relies purely on push notifications. Less wake time, less data use, less battery use. It's similar to the Facebook and Twitter apps. They both have a sync option where you can sync contacts, and likely pull messages at a fixed interval. This helps when you load up the app that new tweets/posts are already loaded, but at the expense of battery and data. However, you can still turn those off and get instant notifications via push. It's what keeps battery use of Twitter and FB (more recently) to a minimum yet I'm able to communicate with friends effectively. Right now looking at my wakelocks, I see G+ and Gmail eating up my wakelocks. G+ especially is a PITA. I barely use it, and I'd like to keep the few notifications I get on a weekly basis on, but the only way to get them is to continue syncing.
Anyway, that's just a thought. The other half of battery life is really the user. It's unfortunate, but people need to be taught to look at the Youtube app and disable "Improve suggestions" or any of those background data consumers. Pulse syncs every 6 hours by default. The AOSP News & Weather app syncs every 6 hours by default too. That includes a location ping too.