Well, usually when someone says "year end mashup", the first thing that comes to mind is DJ Earworm. Year end mashups are megamixes, usually including anywhere from 25 to 70 songs, of songs either charting that year, or released that year. There are typically two styles used for year end end mixes. There is DJ Earworm's: picking the top performing songs of the year and making an entirely new song, with lyrics and a new instrumental. There is also Daniel Kim's (Pop Danthology): Using more than 50 songs, sorting them by key and BPM, and creating different sections which transition into each other. Some other artists who make year ends include DJ Drybones, DJ Earworm, TheDMcI123, and myself