I am underway in a full-progression game and chose the Summoner. Our DM has been gracious enough to allow us to change things about our character until the next level, which has let me fiddle with a base Summoner and Master Summoner, to an extent.
I love the idea of my Gnome Summoner using his Eidolon as a mount, which works fantastic as a Master Summoner. I am the quintessential trickster Gnome who is also the party's diplomat and merchant. Our party consists of the following:
(core)
Gnome Fighter
Elven Bard
Elven Magus (staff)
(come and go)
Half Elven Dragonrider
Elven Druid
Now, with all that being said, our games are RP-heavy and the characters don't focus TONS on optimization (except maybe the Dragonrider). So, I was going to focus myself/Eidolon on a buffer and skill monkey. Since I'm a bit of a tinkering Gnome, I thought the idea of having my Eidolon as a vehicle, would be fun. Starting off, it would be a four-wheeler of sorts. Then when I take the double evolution of Flight, it could be a jet-looking contraption. All the while being sentient and acting almost like K.I.T.T. I even thought the wheels could protrude spikes for a "claw" attack. Or taking a breathe weapon for the jet's "bombing runs". Each customized at the every level or at the cast of transmogrify. Heck, maybe a submarine if the situation calls for it.
So, the advice portion comes here. Firstly, it seems like it would be difficult for me to make him the trap-monkey due to lack of appendages. Maybe a knowledge, sense motive, etc. type focus? Or could you think of a clever way for a vehicle to disable a device.
Second, since this is a roleplaying focus, what are your thoughts on using a full-progression Eidolon for many fun skills/abilities, or going Master Summoner and having a weaker ride but being able to have those handy-dandy summons out at the same time?
I also thought about talking with the GM about taking feats of bow/crossbow proficiency and having a ranged weapon built into the vehicle. So if I went base Summoner, I could do something besides cast my limited amount of spells during combat.