Customizable illuminated signs can be a new massive application for 3d printers, low cost printers and cheap materials as PLA, plus led strip you can obtain a fancy illuminated sign
If you choose put your sign outdoor , take care of sunlight and moisture because they can degrade it
Step 1: Easy and Customizable Design With Sketchup
From a 3d text obtained from standard tools in sketchup and using "push-pull" and "offset" tools you can easily design your lighted letters.
We had success with 4mm of space for led strip and 10mm of height plus 5mm of text thickness. You can also trim the led-strip stubs to minimun in order to get higher area illuminated inside of letter.
Real limitation for your signs is your printer plate size ;-)