Reddit mentions of Tallarico's Hot Hoagie Spread 12oz.

Oh man, you know what makes an amazing italian sandwich? This stuff. No sugar, 1 tbsp is 1g net carb. It’s so damn good, I used to love it on sandwiches. Your post reminded me how awesome it is, I might need to go pick some up and add it to your recipe. Thanks for posting!

Hummus is a great idea and it'd make the sandwich more filling, too. Other ideas:

• add sriracha and a bit of lime juice to the mayo.
  
• Giardiniera, which is usually a spicy mix of pickled vegetables, sport peppers and olives. The finely chopped kind would work better for sandwiches.
  
• muffaletta spread, which is similar but more olives and usually not spicy
  
• Italian dressing (or just a mix of red wine vinegar and olive oil, like they use at Subway)
  
• I think it's more of an east coast thing, but sub places put a spread of crushed red peppers on their subs that adds a lot of zing. Something like this.