One of the easiest and best ways to spice up an existing campaign that is starting to feel repetitive is to invite a new player to join the group. The new player will perturb the existing player relationships and the new character will disrupt the existing character dynamics. This will instantly spice up the campaign and allow new play patterns to emerge. The changing dynamic will improve the game for everybody. Even if the new player can't join the campaign permanently, they will change up the status quo that has led to a stagnating campaign. Invite them along for just a few play sessions and couple this with an appropriate length side-trek or campaign interrupting adventure. They don’t need to be a third wheel if a new narrative arc is just starting. And a little advance planning will even let you lay in several long-term narrative threads that can enrich the campaign tapestry later.
I used this technique to good advantage with a a guy who wanted to “try out” RPGs without committing. It wasn’t his thing, but after a few sessions the old gang felt like a new campaign.