Beamdog said back in 2014, not long after the release of Icewind Dale Enhanced Edition, that releasing an EE of Icewind Dale 2 would be a "nightmare." The changes from the original to the sequel "are deep," co-founder Trent Oster said at the time, and would "require a pretty thorough rework of the entire game to bring in the features from our Infinity Plus Engine." Since then, he has apparently changed his mind, but as he recently told Kotaku, there's another, far more intractable problem: The source code is missing.
“We’ve searched all the archives we have access to, including all the data handed over to Wizards of the Coast from Atari and there is no source code for Icewind Dale 2,” Oster said. “We’ve reached out to our friends at Obsidian, as many of them were the development staff behind Icewind Dale 2, and they do not have any source code. We’re stalled on the project without source and the project won’t move forward until we can find it.”
The source code for the Baldur's Gate games, Icewind Dale, and Planescape: Torment was available directly from BioWare, with some assistance from Wizards of the Coast. But the IWD2 situation is more complicated. “Icewind Dale 2 was created by Interplay working off a customized version of the BioWare Infinity Engine,” Oster explained. “They took the Infinity Engine variant they used to ship Icewind Dale and started from there. Somewhere along the path of the Dungeons and Dragons license moving from Interplay to Atari and then reverting to Wizards of the Coast, preserving source code and transferring it to the new rights holder somehow data was lost.”
And without that data, the whole thing is basically dead in the water: Oster said it is "simply too expensive to attempt to reverse engineer from an existing version of the game data," and so Beamdog has "moved on to other things until there is a change in the situation."
The good news is that Icewind Dale 2 Complete remains available for purchase from GOG, and you can mod it up with various enhancements (including widescreen support) if you feel like putting the effort it.