Modular assets aimed at improving iteration, while not providing a “Game-in-a-box” solution. One of the “problems” with , and the asset store in general, is that it caters to a HUGE amount of shovelware, and detracts from actual game development, and in turn, more assets are created to cater to these needs, and a system is inadvertently created not to suit the needs of developers, but those that throw something together to flood “X” app store with a clone of X trending game.
I think Manufactura is a great example of the type and quality of assets that would be beneficial to Unreal Developers.
Animation that can be easily extracted to fit an existing skeleton through something like Motion Builder would be . Something akin to megascans - a huge library of high-res seamless texture would also probably be hugely popular. Advanced shaders - portals/warps, polish could be useful.