I am strongly against it. I have said many times why. Many of the heroes from the first and second parts are non-canonical because they did not participate in the plot.
And it is very easy to make a non-linear campaign. Two campaigns.
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You need to make it for In the Wake of Gods, as the Roland campaign seems to have Lord Haart and Roland, who cannot be lost, and this can be done in the third game using ERM.
So what? Do all the heroes have to participate in the plot? Then tell me, what role did Sir Mullich or Vistan play in the plot of the third part? :)
No. You can lose Lord Haart there. He does not affect the plot in any way - he is just a "named" hero.