Loial convinces a large group of Ogier to remain to fight TG, while the rest of their race open the Book of Translation and leave.

Jordan could have gone with either, it very much depends if he wanted an ending with Loial leaving with the Ogier after they all fought in TG (and many died, no doubt), or an ending where Loial (with Erith etc.) spends the rest of his life in this world with his human friends, a big sacrifice he and his group make to help defeat Shai'tan. It might even hinge on the possibility he wanted Loial's group to go with Mat to Seanchan and join the remaining Westlands Ogier to the Gardeners when he came up with the sequel idea. The fact he intended to go to Seanchan in a sequel may have made him alter his plans for the Ogier a bit... or not. We'll see.

After TG, there's not enough Ogier to sustain the race that, the reader will understand, will almost certainly disappear in the next millenia, becoming legend etc. As it is, the Ogier race is in decline since the Breaking, with fewer youths in each generation.

Some think the Book is an Harry Potter-style portkey. Personally, I suspect it's a book from the time the Ogier arrived that explains how the Ogier came to this world, and maybe why or how it happened, and how to return... and one unknown is if the Ogier form the whole of their race, or just the part of it that translated into this world, while the core of the race is still in their original world.

It could mean that even the Ogier who fought in TG might be able to join the others later on... well, in theory anyway, as we don't know what the translation process will entail, and if it's a one-time thing only or can be reapeated gradually, first for the Ogier who wish to leave before TG, then for the survivors and eventually the Gardeners in Seanchan.

For all we know, channelling is involved (ie for eg: the Ogier came as the result of some Aes Sedai experiment with the other worlds in the beginning of the Age of Legends) and it will be up to Egwene and the male channellers to agree to help the Ogier to leave. If Ogier Singing is rather involved, then it's pretty much up to Loial who is one of the last with a decent Talent in that area. Other possibilities involve some secrets we don't know yet of concerning the Portal Stones. Jordan indirectly revealed that either the Finns can use them, or they have another way to reach various worlds other than their own and the human one (because when he gave his explanation on the wording of wishes, he let slip that if you ask for something silly, out of their power, like being made King of the World, they could transport you to an empty world where you would de facto be King. It means, they can access other worlds, one way or another...). The fact the realms of the Finns became connected (or collided) with the human world at the ToG may be indirectly linked to the fact the Stedding have also "collided" with the human world at some point (it's fairly obvious from the Longing era that the Ogier can't make Stedding, it seems Stedding can't be made, that they are parcels of their original world, with different "rules" much like the Finn's realm, that translated "as is" with the Ogier. The "random" nature of the Stedding, a lot here, none there, more in Seanchan etc. speaks of "an incident" of some kind causing their translation from one world to another).

I agree it's pretty obvious Loial will manage to convince Ogier to fight with humans and that the Book of Translation will be opened at some point, but this story line is not much predictable beyond that.

It's a bit unlike Jordan to throw in without a world building reason the fact the Ogier have come into this world one day and must leave it at some point. The "why" of all this is a huge missing piece of the puzzle, and with RJ there's always a why. The Ogier obviously think this is part of some cycle/the Pattern that repeats at each turning, like the Bore etc. I'm pretty sure there's a greater purpose to the Translation of the Ogier that will be revealed in the finale (the Ogier themselves may not even know, this may be explained in the book of translation only). There's a reason why they came, and there's a reason why they must leave.

I would not be surprised their presence is linked one way or another to the presence of the Finns or the Portal Stones (the only in-between worlds portal we know the reason for are the Ways... created using the results of research on the Portal Stones...) My suspicion is that the departure of the Ogier (and their stedding) is linked to the "healing of the bore". I suspect that the event that brought the Ogier and the Finns in contact with this world is linked to the apparition of a weakness in the Pattern that slowly built up until at the end of the last Age Beidomon's team found it and bored into it. There's also the fact the Portal Stones were made by those who "knew the numbers of chaos", and chaos has a very bad name in WOT. The Portal Stones may have caused a collision of some kind between worlds, opening a path to the realms of the Finns (who put a door on this bore, ie: the ToG) and translating bits from the Ogier realm into this world (with Ogier in them). It may be that the Ogier must leave, so one day the worlds can collide again and this "chaos" can cause the weakness that will be bored into, and similarly it may be that the real reason Mat must face the Finns again is to close their connection with this world, and the Stedding must disappear (and the Ways must be totally closed off), so order between the worlds is restored and the conditions are there for the Pattern to return to its pre-weakness state, with the worlds only connected through tel'aran'rhiod, before the cycle happens again in the next turning.

The connections to other worlds may be part of the "rubble" Rand needs to clear up to seal off Shai'tan from Creation.

In any case, I'm pretty sure it will turn out to be a bit more complicated than Jordan wanting Ogier in the story, so he made them come and he will make them go without a deeper rationale connected to his world building and his Pattern behind it.

The Portal Stones's geometric symbols are also very much like the Finn symbols on the Doorways (even the fact the ToG is opened by drawing two symbols is a bit similar to how the PS work). It may well turn out they were the ancient race who built the Portal Stones.