But there's this big, actually, really HUGE wet bit in the middle called “the Air-Sea Gap” (i.e. lots of LONG distance flying over nothing but open ocean), plus neither country has the ability, nor desire to invade or interfere with the other (both are democratic now).
Also, recently, at the present, and for the projected next 20 years, there’s no contest for A2A or A2G; Indonesia has one of the weakest air forces in the SEA region, and the RAAF is easily the largest and most capable in the region. (plus everything is being done to improve relations and regional stability – only persistent low-level TNI interference in East Timor’s stability could be a point of potential conflict – and airpower use would be extremely unlikely to occur even then)
Personally, for a future Lock On theatre, I think Taiwan v mainland is much more flight-sim theatre and scenario ‘friendly’, in terms of the hardware in service, and the naval and missile dimension, plus, not too much flying over the wet stuff, plus a real sense of specific territories and motivations, and a 'frontline' to defend, and a drawn out battle potential (which was/is so vitally important within all the best of previous sim titles).