Somewhere in the middle of the run there's generally a Big Two-Parter. This is basically one of the other formats on a bigger scale with a cliffhanger. In particular, it's most often a Secret Invasion, a bigger version of Aliens Of London which involves more sneaking at the start and a bigger fight at the end, different enough that it gets its own entry. But a large enough threat, a complicated enough story, or a fun enough enemy, can be reason enough to go Big Two-Parter with another format, or even something a bit outside the format - Series Three had two Big Two-Parters which were an (Insert Ominous Word Here) Of The Daleks and a tragic romance Pseudohistorical, and bolted the Secret Invasion on to the Season Finale.
All you really need are bigger stakes and a good cliffhanger.