While a simple "I love you" expresses the sentiment in a short and sweet manner, sometimes putting a little more into a text message does wonders. "You are my world" or "you made me see what love really is" are both examples of good romantic text messages.
A real romantic text comes from the heart. "I can't wait to see you" or "I can't wait to spend my life with you" may not be waxing poetic, but the truth behind them makes them something.
Quotes from famous poets or songwriters can also make for good romantic message ideas. "My heart is ever at your service," a line from Shakespeare, is a wonderful way to show devotion through text.