It's now confirmed that expert mode does not increase the difficulty. The only thing "expert" about it is that the player themself has to take control over the balance in the game, so the only reason not to activate expert mode is if someone wants to experience the biggest flaw of the original game. Also, expert mode can be used simultaneously with casual mode, just in case that someone wants to know if it's possible to use all available options.

Makes sense that expert isn't higher difficulty, because the original game's toughest bosses were already scaled to require in-depth preparation.

EXP that is earned from sidequests is automatically applied to your party, so it happens quickly that you are overleveled. There are sidequests with clock icons that stand for timed quests; these expire once you've progressed the story past a certain point, but the game doesn't tell you when that point is. That's why it's likely that players who don't want to miss out try to do a lot of sidequests and that inevitably results in being overleveled for large stretches of the game.

Another drawback that comes with this is that EXP, skill points and talent points scale in relation to your party level compared to the enemy level, so being overleveled means pitiful skill and talent points. This can bite you in the butt late in the game because you'll have underdeveloped skills and talents once you stop being overleveled. A sudden sharp difficulty spike on the final stretch of the story is the result and it's common that players have to grind in order to make it across the finish line.

So I highly recommend to enable expert mode as soon as you have control over Shulk at the beginning of the game. All the EXP you earn from travel points, landmarks and sidequests will be put in a pool that you can always access when you feel that the game got too hard. You can even adjust your level down if you want; in return the EXP goes back into the pool, so you don't lose anything and can play the game at just the right difficulty for you while not having to worry about sidequest completion throwing off the difficulty balance like in the original release.

As practical example, Shulk and Reyn are at level 2 after the first couple of forced battles. I enabled expert mode and reset their level to 1 and remained at level 1 until I set out to do sidequests around colony 9, the starting area. In the original game I was at level 5 at the same point due to all the travel points and landmarks you activate early on. Expert mode made the initial couple of hours already a lot more satisfying for me. And like I said, you are free to adjust your level up and down, so if you felt level 1 is too hard, you could begin the game for real at level 2-5 too, because that's the amount of EXP that will be in your pool.

EDIT: Projected level after clearing out colony 9 in expert mode: 11. Original game for comparison: 13. Projected EXP amount in the pool: Enough to adjust up to 15. So the EXP pool is bound to make me end up with more EXP than in the original, simply because starting at level 1 rewards me with more EXP from battles. Not that I'll intend to use the pool, but for many others the pool will prove to be beneficial if they want to cut down on grinding later in the game.

While I agree it's going to be a great game, I don't think that's relevant to whether it is a remake or a remaster.

I'm not against building up hype for a game, especially one from my favourite dev team of all time. But don't be pretentious by trying to call it something that it's not.

I thought this whole thing started with annoying "It's a remake!" people yelling at me for calling it a remaster earlier in this thread. But apparently they had been yelling at other people about it too, all over the internet, like a raving bunch of lunatics.

Anyway, in the end, reviews confirm it is a remaster, not a remake.

And directly from the horse's mouth, Tetsuya Takahashi describes the remastering process: 

http://www.nintendolife.com/news/2020/05/monolith_softs_takahashi_tetsuya_talks_xenoblade_chronicles_definitive_edition_in_this_translated_famitsu_interview

"Anyways, I'm going to make a general rule that as for this thread, either remake or remaster is an acceptable label for the game, and any further debate over it will constitute derailing and lead to a thread ban. It doesn't seem to be going anywhere, and is just ruining the thread for the majority of people who couldn't care less and want to actually discuss the game. If you feel that passionately about it, feel free to make another topic devoted to arguing that point."

My apologies, I didn't read that.