Hello! If any of you have noticed, I missed writing a blog a couple weeks back, so sorry about that. Anywho, this speculatory blog should quench your thirst for reading random things on the internet. ;) So without further rambling, I will begin.
We all miss the Covenant Empire and it's full strength a little, but with the help with the Didact's forces, could it return? The collapse of the Covenant was finalized with the death of the last major prophet, the Prophet of Truth, but like the Empire from Star Wars, the Covenant could still have efficient and deadly warships 10+ years after their resolve.
Class-CSO supercarriers probably don't go over the 10's or 20's, but only 2 were known to have been destroyed, possibly leaving 8-18 supercarriers (at an all time maximum) still in the Covenant Remnant's control. The Covenant had ships in the low thousands, but by the time of their shatter (including the Elite's departure) they probably only had less then 1000 ships. Even though this isn't hardly anywhere near the original Covenant strength, when you put 1000 ships, each capable of rendering modern day Earth uninhabitable, together they easily could decimate the 2557 Earth which is under reconstruction from the previous war. Many ships were destroyed during the Requiem campaign, but once again, 1000 ships is highly likely to still survive right after the Covenant shattered. From 2553 to 2557, we can say that (including the Requiem Campaign) the Covenant lost a few hundred ships. This leaves us with 400-800 potential capital ships, and that is easily a match for the Infinity and her fleet.
Let's just skip the capital ships for a minute, and move onto artillery/vehicles. In Halo: Glasslands, they discuss this topic quite frequently, with many Revenants and Ghosts littering the populated portions of Sanghelios. If Sanghelios is littered with these vehicles, then many other colonies (even captured human ones) probably have a surplus of these vehicles. Even the large vehicles such as the fully robotic Scarabs seen in Halo 2 or the Lich's seen in Halo 4 could still be in large numbers. If we simply look at the amounts of Wraith's, Ghost's, Banshee's, Lich's, etc, used by the remnant in 2557, and compare that to how many organized mobile forces were seen right before the shatter of the Covenant, we can see how fast the remnant is growing. By the time of the end of the reclaimer saga, the Covenant remnant might even be able to manufacture CCS-Class Battlecruiser's. If they can do this, then vehicles surely won't be a problem to produce. The Harvester seen in Halo 4 even appears to be recently manufactured, so could the Covenant make such large vehicles like Lekgolo Scarab's or Lich's? As well as space vessels, this is both unlikely and likely at the same time.
No army can be made without the infantry, so how many religious fanatics worshiping the Covenant are left? As shown in the Kilo-Five book trilogy, the remnant right after the end of the war wasn't too strong. Not strong enough to become a problem for the UNSC, so we can rule out any major threat coming from Sanghelios. The remnant seen in Halo 4 was pretty large, ranging from 10,000 to 100,000 thousand (or more), bringing up another big question. Where are thousands after thousands of these soldiers coming from, is there more supporters of the Covenant, and where are the last (if any) Prophet's? All of these questions may bring up more, so I will come back to those and/or you can bring them up in the comments. Weapons seem to be the big problem with the remnant, and after five years of waiting in the shadows, the remnant seems to be loaded with enough. The Covenant remnant is only a fraction of what the original Covenant was, so the remnant would need a lot of work in order to be restored to it's original strength. Another big question, what races are still involved in the Covenant? It probably is going to be difficult to find the armor found on Hunter armor, so that armor likely would be salvaged from damaged warships. Farms of Grunts probably aren't going to be feasible, so we can rule out supermassive crowds of Grunts being used as cannon fodder. Jackals? The bird is seen in Halo 4, with some speculation over the species accepting the Covenant's religion. In any case though, we've seen plenty in Halo 4, so Jackals aren't going anywhere. Drones seem to be somewhat independent and self reliant, so they might not have high costs. Where does the Covenant stand on Engineers though? Once again mentioned in Halo: Glasslands, many of the Huragok have been stolen or sold by Jackals. The humans might even have more of these biological supercomputers then the remnant itself.
The Covenant remnant is apparently rapidly growing, so in theory, all of it's fields (Infantry, vehicles and even capital ships) should grow to some extent by 2560, however this brings up another point. The remnant most likely would be destroyed by 2589 (the end of Halo: Reach explains why), and surely would by 2610 (Museum of Humanity opens). In any case, before those times come the remnant will be a pain for humanity.
Please bring up any thoughts on the subject, and thank you.