Rules wise Aura Spells should just check for their Enchant Keywords and these should handle the "can be attached" part.

• The Corner cases: having an Aura become the copy of Old-Growth Troll, when it dies it returns as real Aura and it is gaining enchant Forest . now with two Enchant keywords, both needs to be respected.

• if that's cleaned up, the Aura Spell can be created by CardFactory like other Spell Permanent are.

• Enchant Targets are: Cards on the Battlefield, Cards in Graveyard, Players

• The AI Logic can't be part of the Aura Spell anymore and should probably be either part of the keyword, or be part of an extra SVar.

• cleanup other uses for FirstAttachSpell

• The cleaned up enchant keyword needs to respect Tallowisp that explicit looks for enchant creature, not enchant creature you control

Rules:

15.1b Aura spells are always targeted. An Aura’s target is specified by its enchant keyword ability (see rule 702.5, “Enchant”)
702.5c If an Aura has multiple instances of enchant, all of them apply. The Aura’s target must follow the restrictions from all the instances of enchant. The Aura can enchant only objects or players that match all of its enchant abilities.

An Aura without enchant keyword would be unattached too. (and sent to the graveyard)

Ruling for Immovable Rod

If you target an Aura with Immovable Rod's last ability, it will lose its enchant ability and be put into its owner's graveyard because it can't legally enchant anything. An Equipment that loses its abilities loses its equip ability, so you won't be able to move it that way, but it will stay attached to the creature it's equipping, if any. (2021-07-23)