First. City/Town names have a GENERAL THEME: Colors for Kanto; Plants & Wood for Johto; and sometimes they related to the "versions" of the game like CLOUD NAMES for Unove (because of black and white clouds) or the GENERAL THEME OF THE REGION like Hoenn (connections between elements) and SINNOH (mystical; religiona-related terms). Alola has "Hawaiian adjectives/words" and Kalos has "ingredients for fragrance".
Second, it has to be original: Using musical terms is a bit overrated; I played Pokemon Sage and Pheonix Rising, both use Music-related terms for the names of the locations in their respective regions.
Third, it has to be smooth and memorable: You don't name a town that's hard to pronounce and easy to forget. (Unless that's what you're going for.)
Lastly, remember theres a character limit. You don't name a city, Supercalifragilisticexpialidocious. As far as I've observed the limit to town names is 12 characters (not including the "city" or "town".
Theme Suggestions:
1. Wine, Drinks, Beverages and Spirits (Ale, Cider etc.0
2. Textile related terms (Cashmere, Silk, etc)
3. Vegetables
Special Mention:
Pokemon Spectrum's region, they use DESERT Naming Theme
Pokemon Ethereal Gates, they used a BIRD Naming Theme