I'm also not a fan of permanently missable content. The game that annoyed me the most in that matter was Golden Sun: Dark Dawn. You can miss some items, Djinni or even summons, because they are either blocked later in game or are available in limited events. It's bad design if game allows you to miss something forever, unless there's a point in replaying the game, like having to choose A or B etc.
HOWEVER, that's not the case with Grindea. You will be able to get the items, but just later in game. That's perfectly fine to block or change an area if it's still possible to get the missing content. Instead of punishing players with missing items, they are instead pushed later in game. While some eq can be handy when you get it on first ocassion, it might be useless when it's available later. Still better than not getting it at all and having 99%
What worries me though is that these items are going to be obtainable by "various means". That sounds to me almost like having a sidequest for them. It wouldn't be good if this sidequest was only for these items, because players that already got them will miss the fun. So if they are going to be placed e.g. in different spots than originally, some new item should accompany them, so all players get rewarded for backtracking and exploring.
Now, items aside, there's no real reason to not seal the past, other than sentiment. Maybe some players would want to revisit the area to meet the people again, or there are good sprites you will never see again, or whatever. But do people actually care about completed areas? Do they really revisit them if they know there's nothing there they didn't get? I doubt it.
As for the ability G-Meister mentioned - you don't get one. Time warp is not an ability and it seems like rifts won't probably appear outside Tai Ming. I think they are caused by Zhamla's Crown, which is the third Artifact. I thought that you would be able to control time to raise these small plants, but as someone mentioned, this will be probably done with Spring power. So Tai Ming is the first dungeon so far that doesn't give you anything to help you progress into new areas, other than it's story. There really isn't anything you can miss there.
