Some years ago, my liege (the King of Lombardy) managed to form the Holy Roman Empire as a Hereditary Aristocratic Empire. As an important vassal within the realm, I would like to see elective succession for the empire.

Cannot create 'Elective Succession in HRE' faction

However, I cannot start a faction to change succession type! I can push a claimant, or reduce crown authority within a kingdom within the empire, but no succession changes.

Prerequisites

Empire crown authority is minimal ("autonomous vassals"), I have no truce with my liege, and there are no other obvious impediments.

Which is in line with what the wiki has to say on forming an elective succession faction:

Elective Monarchy in [Title]

• Without Conclave, requires lowest crown authority (Autonomous Vassals)

• With Conclave, requires the council to have all voting powers

So it seems as though I should meet the requirements.

My liege could switch to elective succession

If I switch to my liege, Elective Monarchy is a valid succession law to switch to:

(above is HRE Emperor Inheritance view, not mine)

But it is still seemingly not a valid reason to start a faction.

Previous patches were noted for having "too much elective", and it seems as this was toned down. However, I should still meet the requirements. So why can't I as a vassal push for Elective Succession in HRE?

Succession by Feudal Elective Section in 00_factions.txt

Relevant sections of 00_factions.txt; that is, the plotter scope and target. Wall of text ahead:

# Institute Succession by Feudal Elective
faction_succ_feudal_elective = {
    type = liege_titles
    
    warning_level = 0.7 # At this level of faction power we give player an alert. Set to 0 for no alerts.
    
# Plotter scope
    potential = {
        prisoner = no
        is_ruler = yes
        independent = no
        is_landed = yes
        is_adult = yes
        primary_title = { holy_order = no }
        OR = {
            NOT = { has_dlc = "Zeus" }
            is_voter = no
            liege = {
                OR = {
                    is_council_content = no
                    NOT = { primary_title = { has_law = war_voting_power_1 } }
                }
            }
        }
        NOT = { trait = incapable }
        liege = {
            NOT = { government = chinese_imperial_government }
            primary_title = {
                has_law = law_voting_power_1
                has_law = banish_voting_power_1
                has_law = execution_voting_power_1
                has_law = revoke_title_voting_power_1
                has_law = grant_title_voting_power_1
                has_law = imprison_voting_power_1
                has_law = war_voting_power_1
            }           
            is_vice_royalty = no
            is_feudal = yes 
            OR = {
                year = 1350
                primary_title = { is_tribal_type_title = no }
                NOT = { culture = ROOT }
                NOT = { religion = ROOT }
            }
            NOT = {
                any_demesne_title = {
                    temporary = yes
                }
            }
        }
        NOT = { religion_group = muslim }
        primary_title = { higher_tier_than = BARON }
        
        NOT = { in_faction = faction_succ_seniority }
        NOT = { in_faction = faction_succ_primogeniture }
        NOT = { in_faction = faction_succ_gavelkind }
        
        OR = {
            # Is not affected by a Crown Law title
            NOT = {
                crownlaw_title = {
                    always = yes
                }
            }
            
            # The Crown Law title holder is my liege
            crownlaw_title = {
                holder_scope = {
                    ROOT = {
                        liege = {
                            character = PREVPREV
                        }
                    }
                }
            }
            
            # The Crown Law title allows vassal infighting
            crownlaw_title = {
                NOT = {
                    OR = {
                        has_law = crown_authority_2
                        has_law = crown_authority_3
                        has_law = crown_authority_4
                    }
                }
            }
        }
        
        NOT = { has_character_modifier = faction_succ_feudal_elective_ultimatum_timer }
    }
    
    # Target scope
    allow = {
        is_law_potential = succ_feudal_elective
        is_law_allowed = succ_feudal_elective   
        NOR = { 
            has_law = succ_feudal_elective
            AND = {
                has_law = succession_voting_power_1
                has_law = succ_feudal_elective
            }
        }
        
        OR = {
            is_primary_holder_title = yes
            higher_tier_than = DUKE
        }
        
        holder_scope = {
            NOT = {
                any_war = {
                    war_title = ROOT
                    using_cb = change_feudal_elective_succession_law
                }
            }
            
            NOT = {
                reverse_has_opinion_modifier = {
                    who = FROM
                    modifier = opinion_coerced_into_leaving_faction
                }
            }
        }
        
    }

Omitted are the joining scopes and AI weighting.

^{Note: I don't have Conclave DLC, which changes how council/vassal demands are handled}

It actually requires Conclave as the demands are like this in common\objectives\00_factions.txt:

    primary_title = {
        has_law = law_voting_power_1
        has_law = banish_voting_power_1
        has_law = execution_voting_power_1
        has_law = revoke_title_voting_power_1
        has_law = grant_title_voting_power_1
        has_law = imprison_voting_power_1
        has_law = war_voting_power_1
    }

This is bugged since 2.7.1. I'm not sure whether this has been fixed in yesterday's 2.8.2 patch, if you could post your 00_factions.txt I could probably help you further. If you're not playing Ironman, you could edit this file by creating a minimod fixing the issue.