Participating in the conferences and meetings as well as reading the papers and discussing with colleagues may bring some new ideas.
The first check, at least for those who work in experimental field, is whether it is possible to realize them using the equipment we dispose.
There are also some emotional affinities: some research axes we like to realize more than the others.
Finally, for those who have specific education such as engineer or physician the practical apects may play a role.
How do you choose your ideas?