My motivation for creating missions is to have an enjoyable product in the end. That said, I do it for the joy of others and therefore I am motivated by beta tests. I think you can learn from any beta test: The more negative ones help you to fill any gaps in your own knowledge and the positive ones are a real reward for all these hours building and testing all over.
How do I get ideas for missions? My first source ishttp://www.military-sf.com/resource1.htm
, especially the "Military Targets" and "Missions" sections.
I just take up a keyword from these sections and try to imagine what mission I could create around that, find the proper locations, and decide on which units to use.
A while back, I created the storyline for our OFPEC campaign. While I have toyed with the thought of doing a completely different mission with addons ever since, I keep coming back to the campaign because the broad outlines for a number of missions are already there.
You can find all about it here:http://www.ofpec.com/forum/index.php?topic=32603.0
If I finally decide to use unit addons and a different world other than Sahrani, Wikipedia is a source of inspiration. There's a list of wars:http://en.wikipedia.org/wiki/List_of_wars
where you can just pick the war your favourite unit addon has participated our could participate in.
A more or less detailed description of all the world's armies is here: http://en.wikipedia.org/wiki/List_of_countries_by_number_of_total_troops
And if you are - like I am - into obscure, small armies, you can cross-check
the list of camouflage patternshttp://en.wikipedia.org/wiki/List_of_camouflage_patterns
with the list of assault rifleshttp://en.wikipedia.org/wiki/List_of_assault_rifles
As an example, you can find that Albania uses the US Woodland pattern BIS has already supplied a set of soldiers with and it uses the AK-47.
So, with a lot of artistic license concerning helmets, load bearing gear and support weapons (MGs, RPGs) by giving the standard woodland soldiers Kalashnikovs, you can create an interesting "What if" scenario against the Serbian addons out there. Or against whom you just like to be up against.
Hope this helps!