I got impatient of waiting Sun&Moon and bought Omega Ruby. I was thinking about doing a run with added rules to make things more challenging, but not necessarily a Nuzlocke.

I never done something similar (i was away from Pokemon since the original Gold, until Pokemon X) so i found some rules, but wanted to check if someone has experience in any of these or other rules to recommend.

A few possibilities
- Monotype run: All your pokemons need to have a particular type in common
- No evolution run: can't let your pokemons evolve
- Set Team: can only use 6 (or less) pokemons during the game, usually the first 6 you caught. No starter.
- UU: Only use pokemons from UU tier (does this really ramp up the difficulty of the game?)
- Disadvantage Always: only beat gyms/elite4/rival with pokemon weak to their type (sounds interesting, but also sounds like it would require some planning)
- Underleveled Always: stay a few level below the current challenge level

Any other ideas?

You could do a Pokemon Wedlocke. Its like a harder Nulzlocke

Only use pokemon whose overall stats don't go above 450. Maybe even less. Like 425.

You can do a limited encounter run. Avoid as many NPC battles as you can. This will keep your Pokemon at a very low level.

Been a while since I've played a new one so forgive me if these have been addressed already, but the two off the top of my head would be

1. Stop making trainers battle with single type teams. It's boring, and nonsensical. Using only one type makes you weak by default.

And

2. Don't tell me what Pokémon the trainer will switch to before he actually switches, giving me the chance to just throw out the perfect counter every time

Oh ok, that works. Like I said, havent played any of the new entries in quite some time

Well the last one I played was Ruby, and I don't remember that being a thing

Play Pokemon Go in a Rural area.

lol holy shit ok

Never knew that. Should just be the default though 

Gym battles like Whitney in Goldenrod City that actually require tactics then spaming buttons.

Create gyms that have certain environmental effects such as Rainy Day or Sun or perhaps hailstorm/sandstorm and of course the gym leader pokemon dont get hurt.

Give some of the Champions Legendary Pokemon. Like why cant Red have a Lvl 80 Mewto on Mount Silver lol?

Create mini bosses such as a very high level trainer then usually who appears that you pass to defeat instead of "random power outages'