The following idea is a nerf for some Eidolons (herein after referred to as Big E), and a buff for others.
As the rules currently stand, a Big E with 1 natural weapon gets 1 attack. It gets a 2nd attack at 9th level, with a -5 to it.
A Big E with multiple attacks gets to use them all, without restriction. It even gets Multiattack to help with it.
The problem (if you choose to view it as one) is that building a Big E with just one attack is nearly impossible to do viably. Myself- I don't care for that.
1) If the Eidolon only uses one natural attack type in a round, it gets iterative attacks based upon its BAB.
This means that an Eidolon with just one attack would eventually get more. It also means that an Eidolon with several attack methods could pick one and just use it for that round. (for example, choosing to just use bite and get iterative attacks while giving up claw and/or slam and/or whatever else it happened to have).
it would allow you to specialize in a particular attack. As it currently stands- a single attack mode Big E is sorely below par in damage.
2) You are limited to the number of attacks you would receive by your iterative attacks if you c hoose to attack with more than one weapon mode.
I know I phrased that badly but I couldn't think of a better way.
Basically- You are ALWAYS limited by how many attacks you should have by your bab. Spend as many evolutions as you want on bites, claws, slams, and tentacles. That just gives you piercing, bludgeoning, and slashing damage types to choose from. No matter how many limbs you have or how many attacks you have you still just get a maximum of 4 just like any other creature.
(note, haste and such would work as normal allowing you an extra attack as per the rules for those spells/abilities).
This has the dual benefit is allowing someone to just pick Bite (or claw or whatever) while remaining viable, and also not having to worry about a Big E with 2 dozen tentacle attacks and all that weird stuff.
Yes, I realize this is a nerf to those builds. I think it is a reasonable one, however.