While I do worry about a UI engine either translating to a terrible UI overhaul or a lot breaking for MQ (or both), the reality is EQ's UI is... well it looks terrible to a brand new player. Even before a new player has to deal with EQs learning curve, the first thing they see when they login is a disaster of a UI that looks like it's from a half-developed game. It's nice how much can be customized, but it's a huge pain in the ass to make it look nice, and if you look at 10 EQ screenshots typically 10 of them will look like someone just puked UI elements all over the screen. If I'd never played EQ before there is no way I'd put up with the UI as-is long enough to make it through Gloomingdeep.
So, there's a ton of low hanging QOL fruit, and I'm going to do my best to be optimistic that that's what they focus on and don't just turn it into some unusable pile of shit that is a little prettier.
If anyone from Daybreak is reading this, my two top UI wish list items: snappable/groupable/lockable UI pieces and being able to resize UI windows by manually typing in their pixel dimensions.