So, this is going out on a limb, but based on feedback saying "we need more concrete concepts" I'll try this one:
Name: Ultimate Finisher
General Description: A Pokemon held until late in the game, and then utilized to effect an efficient, difficult to stop sweep after opponents are weakened.
Justification: Every facet of the game is considered in phases - Lead, Early Game, Mid-Game, Late Game. While obviously game state transitions over time such that "Early-Mid" and "Mid-Late" can come almost simultaneously in some matches, it seems to me your Late Game is always a battle of which Pokemon can seal the deal for your victory. This Pokemon is sometimes referred to as the "wincon", or win condition. Since the object of the game is to win, looking at this particular avenue for winning has great potential.
Questions To Be Answered:
What are the qualities that differentiate "early game" and "late game" Pokemon?
What kind of support does as effective late-game cleaner need? Which of the many options (spreading status, setting hazards, etc.) is the best support for such a Pokemon?
What playstyle (6th mon on Stall, Balanced, Hyper-Offense etc.) does a late-game cleaner most appreciate?
With team preview, how does one protect this Pokemon for Late Game, or does it need enough flexibility to be used earlier (i.e. potentially set up first in mid-game, and then come back later.)
Explanation: There have been over the generations Smog articles on the subject of Pokemon Roles, and ORAS has its
own compendium of them. While there's no role for "Cleaner" per se, there are "Wallbreakers" and "Stallbreakers" and of course "Setup Sweepers." Thing is, many articles do refer to a Pokemon as a "Cleaner" or suggest a Pokemon be "held for late-game."
To me then, it seems like this is a Pokemon that is very powerful in many respects but has a limiting principle, say susceptibility to rocks, the need for prior damage, or some other flaw (say low speed) that prevents you from using this otherwise powerful Pokemon earlier in the game. That being the case, I think it would be fascinating to try a build a Pokemon that is literally "the best" at securing this late game sweep while still building in a weakness that prevents it from being generically good. A few Pokemon I think fit this kind of mold are Crocune (difficult to break once things that can OHKO or 2HKO it are removed, but you can't just throw it out there) or the Curselax in UU (or prior Gens). Kyurem-B might also qualify in a different sense in that it can inflict amazing amounts of damage but also Roost to shrug off weaker hits and prevent neutral priority from KOing it.