HAT, or HP - AC - TWENTY SIDED, is a roleplaying game that shares three basic concepts with D&D but diverges from there. The three concepts kept from D&D are obvious from the title: HP, AC, and the D20 roll. However, each is expanded in scope.
This page explains the basics on how to manage creating new Ability traits inside the HAT system. In order to do so, you need to already understand the basic terms.
An Ability trait should conform to the basic model as so, but does not have to:
In addition, you can trade a minor Clause for a modifier increase of +2. So an Ability could offer an attractive adjective and a +2 to STR or an Additional like Trick or such.
What is an attractive Adjective? That is one that has a wide scope (for an Ability), like “all combat actions” or “all social interactions” and so on. A limited adjective has a more narrow application, like “when attacking with a weapon”, or “trying to charm a woman”.
A minor Clause is either costly and powerful, or cheap but weak. A minor Clause may say: “Pay 3 HP to make a single additional attack with a weapon in a round” which is costly but powerful. It could also say: “Pay 1 AC to add the Adjective 'Bullrush' to an attack while in battle.” which is cheap but weak. What is weak and powerful here, cheap and costly, is up to the GM.
A major Clause is cheap and powerful, or free but moderate. To revise the above Clause, it could say: “Pay 1 HP to make a single additional attack with a weapon in a round” which is cheap and powerful. A free but moderate strength Clause could be like so “Earn +1 HP on HP drafts when injured in battle.” It has a condition, but has no genuine cost so its free and has a moderate effect.
A attractive special rule is something really neat that is powerful in the game mechanics and has reasonable scope in effect. One such ability might be: “Pay 1 HP to make a second maneuver after a successful maneuver at a cumulative -5 modifier.” These are really quite interesting things, and there isn't a limit, its just up to the GM. Players should look at the abilities in Creating Adventurers for inspiration.
Here are ideas to expand on the Basic Model with more choices, but these aren't things that are a given. Each would have to be approved by the GM of the HAT game in specific.
These extensions can be seen in some of the Abilties given in the rules, which is why they are listed here, even if the GM must approve their use.