Most good brisket cooks will tell you to learn to cook the meat,first-as that is 95% of the process.
Most NEVER learn,as they get so inolved in the tricks.
That said,I've cooked with a number of the best from around the country,and most agree.
The quality of the meat will mean a lot.
Start with salt,pepper,and garlic,and then some like onion powder.
Most said,add one ingredient at a time,and get some folks to sample.
Most agree that the product should taste like what it is and be slightly enhanced by any flavors we add.
Most feel the taster should go away,wondering what that vague, pleasant blend of flavor was-and not be hammered by it.
Now,it is rumored that Texans never add sugar.
I can't really agree that "none" do.
As to Redoak's thoughts on onion powder,the onion products can all be different in their raw and cooked state.
As an example,science confirms that some folks are "super tasters",and women more often than men.
On our team ,and many folks around us,folks trust my wife.
She can tell I've mixed onion powder in-when she walks in the house.
She is very much against it,and her tasting has paid off.
Many folks feel judges, and tasters in general, like a little sweet.
Heat is something personal,and there can be a real risk in turning someone off,with the first bite.
We,and many,like to balance sweet with heat.
The taster should taste at little sweet -up front,and just a touch of back heat-going away.
There is usually more to lose by turning someone off with a taste strange to them,than wowing them with it.
Some folks may add for color.
Some ingredients may be added for bark,and you wouldn't wish on all products.
Fresh herbs,and stand out flavors, may work better on short cooks.
18 hr cooks may not benefit from either.
Some products can scorch from high heat,or long cooks.
Sugar and paprika come to mind.
These are just a couple thoughts,as we are learning to cook the meat and maintaining the KISS approach.