There's a lot of stuff in the Witcher series that wouldn't translate well to Mass Effect; the Witcher is a carefully crafted story focusing on one very specific individual. Whilst the ME trilogy was based around Shepard there was no definitive Shepard - (s)he could be a soldier or a space wizard, a paragon or a pariah, ruthless or compassionate... the character aspects are totally different between the games.
However, what the Mass Effect series could take on board from The Witcher 3 (not The Witcher 2, that was the weakest in the franchise IMO) would be some of the sense of scale, the use of audio (there are some quite creepy bits in The Witcher 3 massively enhanced by the soundtrack) and, especially, the attention to detail.
ME1 almost has scale - it feels big yomping around a planet in the Mako, despite the fact that it handles like a drunken walrus on a bouncy castle, but you find "commander, you have to turn back, you're leaving the area of operations" all too soon. The same happens in TW3 but it's a much bigger playground... often, if you can see something you can get there - eventually.
The attention to detail in TW3 is a biggy for me, ears hanging on trees, all the buildings being slightly unique (including the wonky building), dynamic day/night cycles and weather, the fact that the environment almost hints at what you'll meet... the way the light catches the water, or how different it looks in calm weather to a storm, "random" whales, corpses hanging from gibbets and (almost) everything has a story behind it. The game feels like life would go on for the inhabitants if you weren't there... especially when you find things like wolf corpses around the elven encampment; shot by a scout.
If they can capture that feeling of scale, the sounds and the attention to detail, the feeling of "life" - where wild "animals" will hunt and kill each other and aren't just there to harass the player... and mash it together with ME3 gameplay and characters... it will be spectacular.
That, however, is a big ask.