Very, very few - and they're mostly 'add ons' rather than changes.
Vision: Rather than just darkvision, I have Darkvision, Low Light Vision (dim light = bright light, an area of dim light exists in double the normal dim light radius), Infravision (think Predator), and Ultravision (very narrowly tailored and useful primarily for arcane purposes).
Flanking: I don't grant advantage for flanking - that is too easy. Instead, flanking is a lockdown maneuver.If you flank an enemy, that enemy provokes OAs whenver theymove (not just when they leave reach). There is a caveat where they can avoid this penalty by giving one of the flankers a free attack (no reaction requirement), but that is there to prvent the 'trivial flank'.
Invisibility: Most effects that grant invisibility also grant the receipient the ability to do a stealth check as a free action when they turn invisible. This is an effective rule change, but not a true one, as I introduce it by creating alternative versions of these invisibility granting abilities and spells.
Potions: Drinking it yourself is a bonus action. Giving it to another is a (standard) action. You can pop potions in your mouth and then swallow them later as a free action, although if they're still in the container, it may cause issues (damage, delays in taking effect, etc...) and you can't talk while it is in your mouth.
Touches: No, not touch spells. Certain things leave a permanent stain on the soul. Death leaves a Deathtouch on the soul. Traveling to a plane leaves a touch on the soul (Going to the Shadowfell leaves you Shadowtouched, the Feywild leaves you Feytouched, the Hells will leave you Helltouched), etc... These touches do not directly impact PCs, but there are a lot of spells, monster abilities, etc... which change if these touches are on you. There is no way to remove any of them - not even Wish.
