I would suggest purinas senior pellet and if the horse is having problems keeping its weight on ( OTTBs are known for this) try purina trimax. It's sweet and they always eat it right up. Doesn't make them hot so when you rider her, she won't be crazy For you. This is the diet my ottb is on, other than senior pellets, just normal pellets and trimax.
With my old horse, a 19 year old thoroughbred, that I only rode once a week, I used to feed him a scoop of senior feed (first soaked a little with water, easier for him to chew) with some canola oil on it (keep him chubby in the winter since he never wintered well), normal hay, and a scoop and a half of soaked alfalfa cubes. Though I am not an expert, but he did well on that<3 Maybe add a supplement for anything she may have some issues with? Like joint problems or hoof problems :)
Depends on the condition of the horse. If she has good muscle tone, fat and energy I'd stay at that. The simpler the better.
Otherwise older horses have a harder time metabolising proteins so I would add amino acids and vitamins.
If she's thin a great fat supplement made of coconut is Powerstance from Stance Equine or even the meal which has proteins etc. ( which I feed all my horses including my 25 and 28 year olds) that is called Coolstance.