This is the kind of thing I think of when I'm bored. If support for triple types and double items and abilities were to be added to Pokémon, what would happen?

Triple Types

It is technically possible to have a three-typed Pokémon on the field now thanks to Trick-or-Treat (adds Ghost) and Forest Curse (adds Grass) moves introduced in X and Y, but by and large Pokémon still runs on the two type system.

Anyway, three types would make it near impossible to pin down a first order optimal strategy, especially for new players. It can already be enough of a nuisance to figure out if super effective Psychic would do more damage to a fighting type than neutral STAB Flamethrower in the sun, and adding a third type would blow that all to pieces. (Answer for those of you who care: Flamethrower.)

Double Items

I don't even want to think about this. Imagine being able to give PorygonZ both a Scarf and a Specs at the same time. Or a Focus Sash and a Liechi Berry to an Azelf. Or both Leftovers [i]and[/b] a Rocky Helmet to a Ferrothorn. Or . . . I'm done now.

Double Abilities

This one, I'm not too sure about. A couple Pokémon have two usable abilities. Both Heatproof and Levitate to Bronzong would take away two of its key weaknesses (and, if you're playing in BW or prior, its only weaknesses). Or Download Adaptibility PorygonZ, possibly giving it a free +1 and an extra multiplier for Tri Attack. Maybe even Blaze Iron Fist Infernape, raising the power of Fire Punch, after STAB, to a blistering 189 at low health. All in all, though, I'm not sure if there would be enough Pokémon and situations to really let this have an impact on the meta aside from sending a few individuals to Uber.

Any other ridiculous scenarios you can invent for Pokémon?

Common triple types would be interesting, but I think at most it could just be reserved for a gimmick mode like Inverse Battle.

Dual items would completely break the metagame, and I'm sure there are a number of combinations that would cause invincibility. Just imagine the Megas with items.

Dual abilities could work nicely if only given to some Pokémon, as Game Freak has complete control over what Pokémon can (legally) have what ability combinations.

Or maybe an item sort of like the Ability Capsule that lets you have both abilities, instead of switching between the two, at least in battle . . .

All of this would make things too complicated. Although, I could see a future ability that allows a Pokémon to have 2 hold items. Certain trade-offs like this are possible.

I have once dreamt of a key item that had the ability to combine two held items into one. When I was thinking of this it was initially for Pikachu because I wanted it to be OP with a Light Ball and an Eviolite. And for abilities, I think an item that switches abilities permanently as a single use item would be good enough. Being able to freely switch, or having two at once, is just crazy.

Unfortunately I can't see triple types happening, as much as I'd like for it to happen that'd just add even more chaos and it'd be too hard to follow.

I once had the idea for double-typed moves, and it seems that's a little too extreme for the metagame, since only Flying Press has ever been capable of being two-typed. I think that these ideas are very doable, and they could add exponential amounts of diversity to the competitive front, but I can only imagine how confusing it would be for new players that they'd have to figure out how effective a Flamethrower would be on a Water/Grass/Ground type wielding an Occa Berry and a Ring Target. Calculating what does and doesn't factor in would probably make a new player's head spin, and that hurts ground-level accessibility into the series, which I believe is the bottom line here.

tl;dr: Would it be good for the competitive scene? Possibly. Would it be good for new players (or even comprehensible)? NOPE NOPE NOPE

The Triple type thing could work. Typing can be manipulated now, sort of.

But before Pokemon gets anywhere near that, any fire move with a base power over 60 should thaw out the Pokemon using it. -_-