I have to admit that I don’t particularly care for some common definitions for missionaries. Some are very narrow (cross-cultural, full-time, church-planter, etc.). I believe that a missionary is primarily apostolic but can serve in at least one of three basic roles.
1. A Missionary Can Serve Where a Church ISN’T. In some places in the world, there is not a viable healthy Christian church so missionaries can take on the role of evangelist, church-planter, leader developer, and discipler. By definition, this role is transitional. If the missionary is successful, this role will not continue (at least at that locality).
2. A Missionary Can Serve Where a Church HASN’T. There may be places where a church exists but hasn’t become involved in important ministries (missions, counseling, leader development, social ministry, etc.). In some cases the churches lack the tools and training to carry out the work, but these are readily available through missionary facilitation. In other cases, the main problem is motivation/inspiration. Again, this is transitional. If locals do not ultimately take over all (or at least much) of the duties, than the missionary has in some sense failed.
3. A Missionary Can Serve Where a Church CAN’T. In some places, churches lack the ability to carry out some ministry functions and are likely to continue to be unable to do so in the forseeable future. This may include things like Christian radio, large-scale disaster relief, and orphanages. In some places, the government or cultural climate limit the roles of local churches. Missionaries may have the autonomy to work because because of their role as an outsider. In this sort of role a missionary may serve in the function long-term. Again, however, incorporation of local people and resources as much as possible is desirable, I believe.
I know that this seems excessively broad, but it does exclude a lot of roles from missionaries. This includes many long-term roles, for example. It also includes roles where the missionary essentially competes with viable local ministries. Anyway, something to think about.