I am sure there are many with great ideas to share. My old ones luckily have no special issues. Allowing me to feed Equine Senior and alfalfa cubes (soaked). My old ones usually have a hard time chewing hay or grasses (unless kept mowed short and tender). Thus the alf cubes soaked for their feedings 2X daily. All mine get electrolytes.
Depends on what issues, do you need to build one up, is there a problem that needs attention or just for maintenance to keep one happy and healthy? I start with getting teeth floated if it has not be done in a while and a good deworming program and then go from there. How old is this horse?
I have all of mine on MVP Exceed 6-Way, even the 26 year old that is mostly a pasture pet. I like to have all my bases covered, plus he still gets annual hock injections to keep him fully comfortable in the cold winter months.