1. Power Leveling
Summons typically appear at a single level, a single echelon of power and ability, and this is generally fixed. An alternate approach is for the Summon Spell to be scalable, and that there are greater versions of the summon's manifestation.
Standard - the summon appears as per the normal description
Lordly Summon - the summon appears as a more powerful version of itself, often with ornamentation and accessories that denote it's higher status. Lordly summons have increased stats, increased abilities, and will gain new abilities.
Divine Summon - the summon appears as the most powerful version of itself, either a godlike entity or a draconic hybrid, the form of power. The summon is much larger, and has auras of divine power and tremendous abilities. Divine summons can only the summoned on a conditional basis.
Servitor Summon - a decreased form of the summon, this allows for the summoner to call a servitor of the summon spell. This is generally non-combatant related. Lordly Summons can themselves chain summon servitors, and divine forms can chain summon standard summons.
2. Stat Boosting
Summons are generally associated with certain attributes, war summons will favor strength and courage, magic summons will favor the mind and perception. As a scalable ability, the summoner can invoke the aura of their summon spirit to boost their related attributes. Being scalable and most summons having more than one key attribute, a summoner can spread this boost across multiple attributes, or pump one very high for a short amount of time.
3. Talent/Skill/Knowledge Boosting
As with stats, summon spirits have skills and abilities related to them, and a summoner can invoke their summons to boost these skills.
4. Hybrid Form
The summoner can attempt a fusion summon, merging their body with the spirit of the summon, creating a fairly longer lasting super form. This is typically much more combat oriented than the average summoner's physical abilities, and will often have supernatural or innate magical abilities that last as long as the fusion is active. Hybrid fusions can be dangerous as it places the summoner and the spirit in the same vessel, and this can have profound impact on the human psyche, can become addictive, or requires greater devotion to the summoned spirit. An alternate use of this power is Congrex, the technical term for physical sexual relationships with non-corporeal entities. This leads to pregnancies, elemental blooded offspring, and becoming magically corrupted/tainted.
5. Boons and Favors
Summons have their own domains, and they can grant boons and favors from this domain. It can be as simple as invoking a hunting spirit to provide a meal, a celestial spirit to grant guidance and clear sight, to as involved as invoking a spirit to provide a spouse, bring back the dead, raise a castle, and so forth. The level of the boon or favor granted is relative to the spell level used, the domains of the summon, and the quality of the relationship between the summoner and the spirits being summoned.
Similar to boons and favors, each spirit will have their own, home, domain. This is where they reside, their place of power. The use of this ability allows the summoner to 'summon' themselves into the spirit realm, and specifically into the domain of their summon spirit. This can be used for sanctuary, transportation, gaining favor from their summoning spirit, seeking information, and so forth.
7. Great Summon
The Great Summon is related to the Divine Form of the summon spell. Rather than summoning the spirit itself, there are greater beasts, members of the ancient race who are associated with the spirit. Invoking this power brings these beasts forth.