Demetrios1453
Legend
I hope you are right.
There is a lot of potential to add lore about one of the traditional celestial races being agitators in the Blood War (I can even see it semi-Planescape style: so you bought into that story that archons just hang around Mt. Celestia protecting it from fiends. Can't say I blame you, they tell it to everyone, but here's the thing, Mt. Celestia is surrounded by an ocean (an ocean I say) of holy water. It is pretty much fiend proof. No, the dark of it is that they spend a lot of time keeping the Blood War hot. The Guardinals are too nice and the eladrin are too hot-blooded, they would make a mess of it, but the archons? They are cold and thorough, the kind of guys that have policies and procedures about what they can and can't do under the banner of Greater Good).
Thinking it over, here are the reasons why I think we'll be seeing the archons and/or guardinals in MtoF, even if they aren't really covered in the main fluff text (besides the one in the previous post, that the subject matter of the books is reasonably favorable for their inclusion)...
1 It's time. MToF will be the third "monster" book of the edition. You only have so much room in an edition's original Monster Manual, so you have to choose what monsters go in there - you have to get all the big name monsters and fan favorites or there will be a big outcry (no matter what you do, there will be an outcry, but you have to design it to minimize that situation). And, whether we like to admit it or not, the Monster Manual is there to give characters something to fight, so you have to, for the most part, focus on hostile foes in the book. Sure, you have a bit of leeway, but if you leave out important hostile foes in order to give a dozen pages or so to celestials, people are going to be annoyed and consider it mostly wasted space that could have been better used. And, face it, parties will rarely get into combat situations with celestials (although there are inevitably evil parties, fallen celestials, tragic misunderstandings, or conflicting orders). And the second monster book, VGtM, was largely, by the designers' admission, monsters they had wanted to get into the MM, but were forced to cut for room. Now with MToF, they should have some leeway to include large groups of non-hostile creatures like the archons and celestials.
2 They would be useful to have. Celestials have the second lowest representation of all the monster types so far in 5e (beat out only by the oozes), so it would be useful to expand on the type - and we have two ready subtypes waiting in the wings. Plus, as they are summonable through spells, and there are so few of them now, detailing more of them gives greater variety for the types of celestials that can be summoned. Also they would be quite useful as party allies in higher-level and/or planar campaigns.
3 They are traditional creatures in the game. Archons have been around since 1e and guardinals since 2e. These are no Johnny-come-latlies to the game, but creatures that have been around for awhile and are familiar with those who have played the game.
4 Space constraints aren't as bad one would think. With around seven subtypes for each, and going by similar entries in the MM for the modrons and yugoloths, they could probably be fully covered in around 5 pages of text each. We have around 130 pages for monsters in MtoF, so even together they wouldn't take up a significant amount of space.
5 Completeness. The Monster Manual gave us demons, devils, yugoloths, modrons, and slaad. We know the eladrin will be in MtoF. So it seems reasonable to complete the alignment set by adding the archons and guardinals as well (although this would leave out the hapless rilmani, who were always the most colorless of the alignment-based outer planar races, in my opinion)...