I make my soup with lots of chopped celery and carrots, and other veggies- adding lots of fresh, chopped or spinach. I also season with mints, cumin, etc, different on different days.

I like to use a chicken stock base and cook the split peas along with diced onion, garlic, celery and carrot, added cumin, bay leaves, a bit of cloves, letting it all cook down for about an hour until the peas are soft. About 20 minutes before it is finished, add more diced carrot, celery and onion, so that the finished product had some recognizable (but cooked) bits of vegetable.

I also serve with croutons or shredded cheese on the side- both items improve the protein usability of the peas. Other garnishes, finely chopped scallions, crumbled bacon, chopped parsley or coriander/ cilantro.

The fastest way to improve the protein is to add nonfat dry milk dissolved as part of the liquid- it complements the pea protein and makes it more digestable. Adding corn- as cornmeal to thicken, as creamed corn or a side dish of cornbread also improves the protein absorption.

Of course, the simplest variation for pea soup is to make the pea soup with navy beans or black beans, perhaps adding some diced tomatoes.

You will get more ideas here:
www.roadfood.com/Forums/forum.asp?FORUM_ID=23

Here is a basic recipe for 6 gallons of pea soup. I vary it a lot, using many times this amount of vegetables.
Ingredient* Weight* Measure* As purchased
PEAS,SPLIT,DRY 7-7/8 lbs 1 gal 1/2 qts
PORK,HAM,CURED,CHOPPED 2 lbs
WATER,COLD 25-1/8 lbs 3 gal
HAM BROTH (FROM MIX) 4 gal
ONIONS,FRESH,GRATED 2 lbs 1 qt 1-5/8 cup 2-1/4 lbs
CARROTS,FRESH,GRATED 1-1/8 lbs 1 qts 5/8 cup 1-3/8 lbs
BAY LEAF,WHOLE,DRIED 1/8 oz 4 each
SUGAR,GRANULATED 1-3/4 oz 1/4 cup 1/3 tbsp
PEPPER,BLACK,GROUND 1/8 oz 1/8 tsp
Method
1 Pick over peas, removing any foreign matter. Wash thoroughly in cold water.
2 Cover peas with water. Bring to a boil.
3 Prepare broth according to package directions. Add broth, ham, onions, carrots, bay leaves, sugar and pepper to peas.
4 Bring soup mixture to a boil, reduce heat; simmer gently about 2-1/2 hours or until peas are mushy. Remove bay leaves.
5 Whip until mixture is smooth. Add boiling water, if needed, for a thinner consistency. CCP: Internal temperature must reach 165
F. or higher for 15 seconds. Hold for service at 140 F. or higher.

And here is a recipe for 10 gallons Bouja, a brothy meat soup:

7 pounds boiling beef
2 heads celery, cut up
1 head cabbage, cut up
8 onions, cut up
6 (12 oz size) packs frozen beans, corn, peas, carrots, etc
2 (15 oz size) cans rutabagas
2 (15 oz size) cans lima beans
2 (15 oz size) cans cut yellow beans
2 large (28 oz size) cans whole, peeled tomatoes crushed
salt and pepper to taste
1 ounce dry pickling spice
8 gallons, approximately, rich broth or stock

Directions:

Boil chicken and beef in a heavy kettle until tender. Use enough water to cover. Remove meat and cut into small bite-sized chucks. Discard fat and skin. To broth, add celery, cabbage, onions and boil until almost tender. Add remaining vegetables according to taste and desired thickness.