Invokers are not the best summoners in the game, but their key abilities (Intelligence, Wisdom and, to same extent, Constitution) allow them to dabble in other power sources.
Before going multiclass, you may want to try Righteous SummonsDDI, which gives you a summoned-creature related benefit according to your Covenant.
There are also a handful of good Artificer, Druid, Wizard and Psion feats but they are really class- or power-source-related.
High Intelligence allows you to easily multiclass as a Wizard. Wisdom makes you a good Druid too.
Of these classes you will be intrested more in their paragon paths than in their class-specific feats. However, you could find tempting to enlarge your summoning arsenal with multiclass power-swap feats.
Also, multiclassing to Wizard lets you use some magic tome implement that enhances summoning (even if only some of their properties are transferable onto Invoker powers).
The Clockwork EngineerDDI paragon path is too tied to Artificer-only summoning for being a viable choice.
If you like the natural side, maybe Primal SummonerDDI could be a really good choice. Druid's summoned creatures usually have a good instinctive effect that allows them to go in autopilot mode while you have more important business to do.
- 11th: You issue a command to one summoned creature (regardless of power source) when you spend an action point.
- 11th: A Wisdom-based encounter attack power that deals damage and makes your summoned creatures (regardless of power source) deal more damage to the target for 1 round.
- 12th: A daily stance that gives a +2 attack and damage bonus to all your summoned creatures (regardless of power source).
- 16th: When you move, you can move one of your summoned creatures.
- 20th: A good summoning power that changes form and may act automatically (instinctive effect bonanza).
Your summoned creatures also heal and receive an attack bonus when they use instinctive effect (but you'd probably have none until 20th).
The Master SummonerDDI paragon path has good features and a nice 20th level summoning power, but it relies a lot on Charisma, and the buffs it provides are only for creatures summoned via Psion powers.
AlienistDDI is a totally Intelligence-biased paragon path, but its summoning flavor is limited to only 2 bullets:
- 16th: Creatures you summon (regardless of power source) receive a +2 to attack rolls and deal +5 psychic damage. They also have a minor drawback (grant combat advantage against fearless opponents).
- 20th: A daily summoning for a creature that deals a lot of damage, but it has a close burst 3 instinctive effect that could be a pain for you and your allies too.
Bonded SummonerDDI is the Wizard paragon path here. It gives you a bunch of tricks:
- 11th: The ability to issue a command to each of your summoned creatures (regardless of power source) when you spend and action point.
- 12th: An encounter utility power that heals a creature you summoned (regardless of power source).
- 16th: Creatures you summon (regardless of power source) gain regeneration 10.
- 20th: A good summoning power that calls a large creature with nasty blast attacks.
You also get the ability to teleport 2 squares when you use an arcane summoning or conjuration power (11th level) and a good Intelligence-based battlefield control power that scatters enemies away (also 11th level).