Which platform ?

This is already kinda possible by creating a POI in the GTK GUI (available o the N900 & desktop) and then going to the POI details menu and using the "set as position" option. Not very discoverable and needlessly complicated.

Something like long pressing a place on map and selecting "set as current position" or being able to directly input coordinates should be also available.

At least on the N900 this happens as the system-wide location subsystem narrows down you position - first it finds out the country, then the region, etc. until it can reach a GPS based 3D fix. And unfortunately as it founds out the country it first zooms at the "center" of the country, resulting in the initial fix being in the middle of nowhere, until finally jumping to the actual place where you are.

I think I might already have a RFE somewhere for checking the fix quality and just discarding these spurious fixes until the correct one arrives (IIRC it can be detected from the fix metadata).

That way modRana would just show the last known position and then would jump to the actual GPS fix. Having a set "home" position might that would always be the focused when modRana starts up might also be useful. The concept of "home" coordinates could be useful also elsewhere, like "route home from where I am", etc.

This is where the modRana Monav offline routing data is stored:

http://data.modrana.org/monav/

also check out the modRana offline routing guide about how to use the routing data packs.