Two years ago at Heidelberg we practiced this kind of self limitation.
I am rather a wide angle guy, so I went for something new by choosing the Voigtländer 90 mm. It was a very rewarding experience.
In Scotland I only used 25 and 50 mm - less limiting, but totally sufficient 90% of the time.
Choosing now, 35 mm would be the most obvious, the most comfortable choice.
But if you really want to gain a new perspective, something more ungainly is recommendable.
I'd probably go for 25 mm or 90 mm again.
I'm not masochist enough for anything beyond 100 mm....