It is my understanding that the esti -onta often carries the idea of immediate future - so 'about to'. You could make this more explicit by adding a 'tuj'.

So either Ni estis ludontaj futbalon, kiam ... or Ni estis tuj ludontaj, kiam ...

In the Tekstaro: 9 hits for tuj -onta(j); 3 hits for ĵus esti -onta(j)/

Over 300 hits for esti - onta. If you have the time, you could do a count of the number of times the reference is to the immediate future.