Instead of putting an E-Rank, why not have sub divisions for D-Rank instead? Looking at the current mons in D-Rank, there are quite a few noticeable viability gaps between several mons, so having sub divisions would fix this issue as well as allow Pokemon that are a bit more niche than some of the current D-Rank mons to get ranked.
Personally, I'd rather have an E rank on the basis that a lot of the stuff that would go there is a bigger step below anything that's in D than, say, the B- 'mons are to the B or B+ 'mons tbh. Having an E rank also means that there isn't a break in the way that the +, = and - ranks are being allocated (i.e. to the ranks which are considered to be relevant in the metagame), meaning that it is far more fitting for the lower ranks. Remember: the main aim of adding an E rank would be to encourage people to try out new things, and I feel like having a D- rank wouldn't have the same psychological impact as an E rank would tbh.
Anyway, I am going to bring up a few noms.
Firstly, I'm seconding the Mega Houndoom nom. I recently built a team around it and I found that Houndoom is really f*cking good. Hell: Houndoom was able to set up Nasty Plots on a lot of the metagame due to its speed tier+typing, and its ability to catch its usual switch-ins such as Azumarill with Sludge Bomb was the icing on the cake tbh. It was VERY fun destroying opposing Zard-Y teams and, beyond just Zard Y teams, it is actually rather anti-meta. I never regretted not having HP Grass simply because it pairs well with a lot of the 'mons which are able to beat things like Seismitoad and Gastrodon anyway, and I never regretted using Houndoom over Zard-Y even once tbh.
Mega Gallade-->B: At this point, it is very obvious that Mega Gallade and Mega Medicham are equally good at their respective roles while being very easy to compare to one another despite fitting on different teams. They both hit very hard, they both mega evolve from things which are otherwise shitmons and they share their typing. However, Medicham is far more effective as a wallbreaker while Gallade is far more effective at cleaning. Now onto the stuff that isn't blatantly obvious to anyone with at least one brain cell. Neither of them are particularly splashable, with them having both having rather large opportunity cost associated with their use (much like Mega Sceptile in both OU and UU). However, the sheer power means that they have means they are both usable on some teams, but tbh I don't think that Mega Gallade warrants being as high as it is ATM.
Chansey-->B-/C+: I am bringing it up again because I genuinely think that Chansey is not good in the meta anymore. It is so passive that it can't contribute anything to any playstyle other than stall and there are simply better options for a special wall that mean that it struggles to make any major impact in any encounter I have with it (whether I'm using it or my opponent is). It isn't hard to deal with, its typing means that it is vulnerable to the coverage of a lot of the things that it is meant to check and its lack of offensive presence severely hurts its splashability to the point where I simply don't feel that it is on the same level as any of the things that are in B such as Quagsire and Amoonguss (to compare it to other stall'mons) or Slowking (as a special wall/tank) and it is debatable as to whether it is even on the same level as a lot of the things in B- tbh. I have explained in the past that I find Blissey to be better on most teams due to its improved splashability and utility as a result of its ability to carry coverage, but I'm not going to bring it up again atm.
Gothitelle-->B-: I have a thing against Gothitelle, and I simply find that it isn't that useful a lot of the time (and certainly not on the same level as stuff like Mega Garchomp and Empoleon). While Shadow Tag's utility is great, I simply struggle to see why Gothitelle is ranked so high as, tbh, if you are reaching a point where you need Gothitelle there is most likely a pretty big hole in your team that can be filled by changing one or two 'mons. As a result, if I ever see a Gothitelle team, I will look it over thoroughly and usually come to the conclusion that there is a hole in their team. I haven't seen a well built Gothitelle team outside of SmogTours, and having scanned through to find Gothitelle matches from all points in the gen 6 meta I have concluded that it is about as useful as the B- stuff.
Finally, I am bringing up a nom that I made a little while ago but I feel got sidestepped due to the same post containing my defense of the ben gay rank. I'm just gonna quote it because I'm too lazy to type it again.
Jellicent: Unranked-->D
Jellicent has certainly fallen from grace since gen 5. However, it does still have a niche - however small - that justifies use on select teams. Firsty, it counters many aspects of rain with its typing, bulk and ability. It fits onto a select number of stall and defensive teams as a result. In addition, Water Absorb+Will-O-Wisp allows it to set itself apart from a lot of the bulky Water-types that are in the tier, and it also has reliable recovery in, well, Recover. The main (and only) set it should ever run is SpD, as otherwise you might as well just use Seismitoad. However, its small niche comes from it a: countering aspects of rain, b: having access to Will-O-Wisp and c: having reliable recovery over other Water Absorb users. It is small, but I feel that it warrants D at the moment. The biggest problem is definitely the Knock Off (although it can play around Bisharp to an extent with Will-O-Wisp), but it can still function effectively with the correct team support (namely Knock Off absorbers - it pairs v. well with Gliscor).
Also gonna add that it hard counters Keldeo 'cause that is neat.