Personally, I want to divert from the Naganadel discussion just a bit since the consensus seems to be rounding out to (let's get rid of the thing), so let's talk about something a little more... unexpected, but needed.
We've all been arguing on Showdown about this, but a large percentage of the people on the chat and in battles (around 70% from what I've seen) think that Deo-D is deserving of a retest once the meta settles and Naga likely gets the boot. Let's break down why it'll fit a lot better now in OU than it did previously. If anyone wants to throw any additional material onto my main points please do.
1.
Stats: While access to Recovery and 150 / 150 defenses are wonderful, that 50 HP is really letting it down now, especially with the advent of Z-Moves basically honing that in. With the massive Power Creep of OU in full effect as well, it has many new threats while keeping all of its old ones. Tyranitar, Greninja-Ash, Greninja, Scizor-Mega, Blacephalon, Hoopa-U, Sableye-Mega, Weavile, and many others in OU can either check or counter it, or even straight up nearly OHKO it. Not to mention 90 speed is far worse than it was during when its last stint in OU generations ago.
Relevant Calcs:
252 SpA Choice Specs Blacephalon Shadow Ball vs. 252 HP / 4 SpD Deoxys-Defense: 290-344 (95.3 - 113.1%) -- 75% chance to OHKO
252+ Atk Tyranitar (Assault Vest) Crunch vs. 252 HP / 252+ Def Deoxys-Defense: 152-180 (50 - 59.2%) -- 69.9% chance to 2HKO after Leftovers recovery (Good chance to OHKO with Band)
252 SpA Choice Specs Greninja-Ash Dark Pulse vs. 252 HP / 4 SpD Deoxys-Defense: 294-348 (96.7 - 114.4%) -- 81.3% chance to OHKO
252 SpA Life Orb Greninja Dark Pulse vs. 252 HP / 4 SpD Deoxys-Defense: 195-229 (64.1 - 75.3%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Choice Band Hoopa-Unbound Hyperspace Fury vs. 252 HP / 252+ Def Deoxys-Defense: 294-348 (96.7 - 114.4%) -- 81.3% chance to OHKO
252 Atk Kartana Knock Off (97.5 BP) vs. 252 HP / 252+ Def Deoxys-Defense: 140-166 (46 - 54.6%) -- 5.9% chance to 2HKO after Leftovers recovery
You get the idea, and that's even without Z-Moves thrown into the equation, then you'll start seeing a lot more OHKOs. It simply is too pressured with the massive shift in power, and can't really do anything to staples of OU at the moment. Not to mention Mega Sableye (which was originally banned) is the bane of Deo-D's existence, and nearly the perfect counter.
2.
Deo-D didn't get anything new: Probably the worst offender here, Z-Moves basically did nothing for it, and it can't afford to lose out on Leftovers recovery since Recover simply isn't enough anymore. The only new moves it got were Brutal Swing, Throat Chop, and Stomping Tantrum, which don't do anything for it. Not to mention that plenty of old checks now have new ways to play around it. It's also outclassed by Toxapex due to having a better niche, a better ability in Regenerator, and a far better typing. This should be pretty self explanatory.
3.
DeoSharp isn't anywhere near as viable as previously, and it'll increase creativity and team variety slightly: Again, due to the power creep and metagame trends, the combo is countered far more easily than it was before, not to mention it would help de-centralize Lando-T a little bit (which is needed) without being the massive centralizing force it would have been in late Gen 5 / Early Gen 6. Additional hazard control from Pokemon not named Toxapex or other staples would be a refreshing twist, and would help flesh out US/UM's metagame even more.
4.
Massive amount of new Defoggers: Self explanatory, Not only did Defog get a lovely buff since Deo-D's last OU test heyday, but US/UM brought it via a move tutor to a whole slew of new threats that can easily switch in and deal with Deo's hazards while threatening Deo-D with Knock Off or other options.
5.
Crippled by Status, 4MSS has never hurt so bad: Deo-D feels like it has to run a million different options to be able to handle everything in OU, and while it has nice coverage options, it doesn't have the offensive presence needed to help with it, and is usually threatened by SOMETHING on your standard OU team. Not to mention that status still cripples it halfway to hammerfell.
My point? Deo-D won't be anywhere near centralizing, and in my opinion would be a solid A- to A rank Pokemon that would help to flesh out US/UM's metagame variety, and I think is worthy of an OU test.