Traditionally former presidents do not publicly criticize sitting presidents, but these are clearly unprecedented times. I feel like if Obama and Bush were to come out together with a joint statement, possibly when Trump makes a blatantly unconstitutional move like ignoring a scotus decision, it might go a long way and creating proactive resistance.
I would suggest Clinton and Biden but I would worry they have too much baggage. Obama and Bush still command a great deal of respect among their respective bases. And together they would be a visible reminder of the era of normalcy. I just feel like it's going to take something dramatic like this to really wake up most people and also make sure it can't be ignored by the mainstream news media.