Crema or sour cream, pico de gallo, sliced avocado, crumbled Cotija cheese, hot sauce, and sliced red onion are all great additions to shrimp fajitas. Cilantro is a classic addition that adds freshness as a counterpoint to the char, but mint or chives would also pair well.
You could swap the pico for Mango Salsa or salsa verde. Serve everything family-style so everyone can build their own.