A bad thing about location in the plot is the game has internal chapters but does not share these details with the player. The chapters are referenced within in-game files (cutscene voices) and the artbook. If the player was told about them it would be much easier to get an idea of how far you are (though a couple of chapters near the end are super short) and would make discussion far easier. Whenever a system save is the Xenoblade logo that indicates you are onto the next chapter but who counted those?
I'm more curious about where the two alien races come into play there, and if that flash of a guy who looks like Shulk in the first trailer was implying any kind of connection (I rather doubt it, given that Earth can't be destroyed in a universe-ending/-creating flash of light only to somehow exist to be destroyed again in an interstellar war it wasn't even involved in - it's a biiiit inconsistent).
I think it is a spiritual sequel now and plot elements may have changed between trailers.
We do see Noppon and that girl with the Monado earrings in the latest footage so there are going to plenty of references at the very least.
1)Equipment/Inventory. I finally reached a point where I can no longer store new items/gems/whatever. What should I be doing to free up space? Sell off low-level gems? Is there a fusion mechanic I completely forgot about to bundle low-level gems into higher ones? Animal parts and other mats also. I'm overloaded now and its been so long since my last play that I actually didn't know if it was safe to offload them or concern over future use.
First off
there is an |OT| buried and forgotten in community.
Yes, sell off low level gems, the bonuses are crap in comparison to higher level ones.
There is a crafting guide here. I would say it is only important once you are finding level 3 materials.
As for the other stuff like animal items, some of it is used for sidequests (but despite doing a lot of these in both playthroughs of mine my advice is to skip most of them) so sell off common at the very least. The thing with sidequests is they are good if they are no effort but if they are effort they get in the way of progressing the game. I guess you could look at a spoilery quest spreadsheet to cross check if you are worried. I was trying to work on a less spoilery one (I got fairly far in the game too) but these things are a lot of work.
There are too few interesting quests to trawl through the junk (the previously mentioned giants is a good example of something interesting and I don't think that has much perquisite). The worst thing is there is a voice acted one really near the end of the game that is super easy to miss (all they needed was a 5 second cutscene to hint at it, the fact they put voice acting in implies it was important).
2)Team-link combat ability thing. I roll with the starter three for Reyn's tanking abilities and the sniper lady's healing/stat spells. When I build up the meter the game keeps yelling at me to do a team-link, but when I do, I can't seem to figure out a solid set of chain abilities between these three characters to make it effective. I am unclear if you can even switch the order while it is happening to take better advantage of the Break->Topple ->etc... ordering. Just feels I am missing something from the tutorials I did years back...
Do not be afraid to change up your party. The thing about Sharla is you auto heal after battle meaning healing is less important and she has bad damage output* amplifying the need for healing (as you take more damage). Light Heal and Riki's spell are surprising good. Do not bother with Melia's Summon Aqua as that is awful though (it is too little regen to matter).
*-She can do ether damage so is good vs elementals and other enemies that heavily resist physical, there is a dungeon late game full of physically resisting enemies where people get stuck on its boss. The art symbol (the direction of the streak in the background) or description will say if a given art is physical or ether based.
I personally rotate party members to build up affinity (you will likely be far from maxing out in any case) to open up skill linking but you might find a party member you really enjoy playing as. Also the AI does not play characters particularly well (for instance with Shulk they will try to spam monado buster and use the terrible exchange HP for talent gauge refill art, I think there is a way to disable that) though it will try to topple (like it notices a break has been made) and do other complimentary things.
Chain attacks are either going for a topple (if you can get enough turns I guess you could go for dizzy and a headshot with Sharla but bosses and unique enemies are immune to the insta-kill part) or building damage multiplies up for massive damage by going for the same colour in succession. Sharla is a problem here as she has a lack of Red which Ryen mainly is made up of for doing that. It should be noted white talent arts (e.g. monado) are a whildcard so you can change colours with those.
From what I remember status like bleed and burn is affected by the multiplier so going for a x5 finishing off with Dunban bleeding the opponent will really cause a lot of pain.
3)Monado special abilities. Very situational...half of them sit unused for long stretches, making me wonder if I also had forgotten some key competent to their use beyond the obvious(Mechon damage, et al.). Actually it feels like I only really use the damage shield when time stops and warns me of big damage...and even then I am unclear if I am doing it right.
There are various symbols to the attacks. The shield only works on techniques that have a name ending with I-X (where the shield technique needs to be that level or higher to work). Don't feel too concerned with leveling this a lot (it is fairly cheap to level which might surprise how low a level the enemy techniques are for much of the game).
Speed will make you dodge any physical attack as it applies something like a few hundred agility buff to the target for a short amount of time, btw agility is the most important stat in the game as it is hit and evade.
There is a secret one with greatly reduces damage the party takes in general which is a lazy substitute to both of those (is uses more of the talent gauge but you can refill it quickly enough late on to keep this one constantly in use).
Enchant does not need much leveling, battles are not long enough to need anti-mechon for a long time (plus the game does go through lots of periods of no mechon) but if they are just cast it again.