1) If things go too well throw a spanner in the works! When they hold off the Orks have a disaster strike. Certain
NPCs in the Imperial Army are secretly Chaos Cultists! They sabotage the defences to let the Orks rampage through the mountain pass. Your Rough Riders are cut off and need to get back to their lines before they are over run or run out of supplies. They come across wounded and have the choice to take them back or leave them, will this slow them down too much? What about their own rations and medical supplies? Can they root out the heresy in command? Is that rather busty and attractive 1at Officer a Heretic after all?
2) Lack of anti-tank weapons mean the Orks break through in vehicles! The infantry are trapped Stalingrad style in the forward trenches! Can the roughriders charge in and stop the armoured assault in time?
3) Silly but fun! Ork spores cross the lines and suddenly partisan groups of feral Orks and Grotz are roaming around in the rear. Let your players hunt them down and track down the source of the deadly Greenskin producing mushrooms!
4) Western style. An imperial wagon train has to move through the moutains to resupply the troops. The going is too tough for vehicles so animals and wagons are used. Can the players get the wagons through or will they be ambushed and destroyed by the waiting Ork Tribe menace?
5) Take a break. Give the players a chance to play a big battle! The Rough Riders find forward Command destroyed and suddenly they are the highest ranking around. Convince them to map out an offensive to encircle and break the Ork Speartip around on key defensive town. Let them play general and have them make tough decisions. The imperial forces are out numbered and short of supply can they pull it off? Would make a good gamble game with the lives of many units at their hands.
6) Dark days. Imperial forces are loosing. Your unit has to hold a key area. The imperial governor has made a final levy of all available manpower for the front. Apart from the rough riders the only other units available are untested militia. E.g like Home Guard/Volkssturm. This mixed band are either too young or too old. Might be best to use a weak profile like Chaos culitist with autoguns and perhaps a few anti tank units with one shot weapons. Can the rough riders hold the Orks off until proper reinforcements' arrive?
7) Tango Down! A valkyrie has been shot down in No-mans land with a high profile person/intel. Your Rough Riders are tasked with getting them/it back to friendly lines. Watch out for Ork Picketts! Probably best in a dense environment like mountain forest where they can't see the orks but can hear them. E.g. Ork patrol walks right past them and both sides don't notice each other!
8) Imperial Blunder! Due to the lack of trained officers, command is making stupid decisions. They are moving armies that only exist on paper and don't understand how strong the Orks are. The Rough Riders are tasked on a clearly suicide mission to charge the Ork lines with other units. The players can't disagree or they will be shot! To make matters worse a rather bitchy Commissar joins their ranks-something you could play as. They do annoying things to the players that generally hate her. When they charge the lines do they let the Commissar get killed or does she come out of the battle a closer friend than enemy? As with the battle can they pull through or does it turn out to be a rout and a race back to their original lines?
9) I got a brand new combine harvester! Surrounding the mountain range is a great plain used by the planet to grow vast fields of crops for the population and for export to worlds that need it. The Orks are breaking through on a lightly defended mountain pass. There is a large field and farm nearby full of wheat that is desperately needed. (There is a food crisis even with rations!). Your roughriders are tasked to collect as many tonnes of crop as they can before the Orks get it. One can operate the Harvester and the others can act as defence or pull carts full of crop to mobile Silo vehicles. It is obvious they won't get it all. When that happens they must burn the crops and building so the Orks don't get the supplies. Maybe the Harvester driver can combine some Orks in the process with the thresher blades!
10) The Great Retreat/Advance. Depending on how well their operations go, high command utilises their mobility for key missions. If the Imperial Guard is losing badly just like the farm mission, the rough riders are used as a mobile unit to burn, blow up and destroy key installations before the Ork advance captures them in a 'Scorched Earth Policy'. They also need to rescue Refugee groups back to the retreating Imperial lines before the Orks get to them. Or if things are going well, the Rough Riders are used as Recon to map and destroy retreating Ork units. However beware they don't become victims of their own success! Rough Riders outpace the footslogging Guard and they could find themselves too far outside of Imperial Lines and about to face a group of Orks too strong for them alone to deal with!
Well here are ten rough and hopefully fun ideas for you to play with. If you don't like them, then at least it will help you narrow down some better ideas. Hope your Only War campaign goes well!
BTW are you using just narrative or are you using miniatures?