These suggestions have largely come from people's observations of the evolution of Pokémon's mechanics. These are:
Gen I: Start.
Gen II: Held Items, new types
Gen III: Abilities.
Gen IV: (Cross-gen evos), Physical/special split
Gen V: (Elemental gems), Abilities->Hidden abilities.
Gen VI: Cross-gen evos->Mega Evolution, new type.
Gen VII: Elemental gems->Z-Crystals
This gives us some roadmap as to what might be next:
- Abilities: Currently biggest advancements to abilities have been HAs and Mega Evolutions, as well as form based ability differences within a species(Basculin, Meowstic). However, as of now, HAs are on the road to becoming obsolete, which means the road to "Ultimate Abilities" is now open.
- Types: Biggest advancements to types have been physical-special split and recent type-based immunities, such as Prankster immunity or Spore moves immunity. While regional variants have created the idea of alternate types alongside Megas, a full on change in mechanics have not come. Most certainly there is no "Ultimate" form of types.
- Held items: Held items have gained and continue to gain subgroups, but no major change for items have happened, besides form changer items and Z-crystals, which are key items that can work as held items.
Abilities: An Ultimate variation of Abilities can work with a single item, an Ability Booster or a similar mechanic to Z-Crystals. Abilities themselves do not have their canonical subgroups, similar to how Moves and species have, so there is not much means of making this incredibly marketable. This will likely happen in the future, but it will likely take some time.
Types: An "Ultimate" variation of types can occur in several different methods, and individual species and subgroups can have their own subgroups. Such a mechanic will likely be introduced in order to replace type enhancing items, which are now obsolete.
Other mechanics such as type changers and type adders can also work with or without fleshing out.
Held items: An "Ultimate" version of items can only occur through an item upgrader of sorts, which cannot be held by individual Pokémon, but used as a mechanic or key item. This will be largely relevant to signature items, however.
Other proposed items include a full on body armor, similar to those in Insurgence.