So, I'm old fashioned and I like the "thought balloon" effect since I work in a retro comic strip style. But using too much thought balloon can make the page too busy, so I use a hybrid normal border that shifts to a partial balloon just to hint at the idea. Although, it's a little tougher when the strips are arranged vertically.
Fade to: https://tapas.io/episode/2375807
Fade from: https://tapas.io/episode/2380739