I suppose I should mention at this point that you guys don't [i]have[/i] to have a Heresy occur just because that's how it went in canon. Indeed, the main reason the Emperor-fragment did its whole timejump-soulmerge thing was to try and avert a Heresy in the first place. There will likely be events that attempt to induce corruption in one or more of the Primarchs, of course - such is the nature of this setting - but the ultimate tone of the setting is up to you, based on the actions your Primarchs and their Legions take.