You can add torches, lanterns, lamps and countless other sources of light, but there will always be problems of having to change up your build aesthetics in order to get proper lighting. This isn't only useful to prevent mob spawning, but also enhances the looks of these builds.
I propose to add invisible light sources, much like we have barrier blocks. In order to see the invisible light source you would need to be holding it. Perhaps the item could just be a 2d lightbulb. Unlike barrier blocks this "block" can be walkthrough.
Only problem I see with this is not being able to build in an area that has one of these light sources. Maybe you could place a block in them which would overwrite them(much like placing a block in water).
I do understand that part of the charm of Minecraft is being creative with the limits that are given to us. This "block" should therefore be a creative only block used for map makers.
The Meaning of Life, the Universe, and Everything.
People always like this suggestion. While I'm a fan of the idea I have to figure that if something people have been requesting and suggesting for over 7 years still hasn't been added to the game then it's likely because Mojang doesn't want to add it.