With my first pregnancy, I knew from about 12 weeks that due to problems, a C-section was going to be compulsory no matter what. The docs were aiming for 38 weeks but I had several near miscarriages with him and eventually an emergency Caesar at 35 weeks. Because I knew this from very early, my birth plan was a bit odd. It didn't cover labour at all, as I wouldn't be allowed to go into labour, and it covered preferences on anaesthetics and what I wanted if an emergency hysterectomy became necessary and on blood transfusions. But for all of it, I put that I would trust the obstetrician's judgement as that seemed sensible for my circumstances. In the event, they refused me the epidural I had down as my preference as I was bleeding at the time so they went straight for a general instead - I didn't argue, I just wanted whatever was safest for me and baby and I had a lot of faith in my obstetrician, he's wonderful.
All went well and I'm now due my second in October. I'm not allowed a VBAC so will probably have a planned Caesar at the end of September, assuming this baby is more patient than my first! My birth plan will probably be similar to last time - preferences for anaesthetic, feeding baby etc but nothing too rigid.
You're right to include the Caesar in your plan, and hopefully thinking about it in advance will mean you feel less disappointed if you need one again.