Well changing the eidolon back is easy enough, but how to get a druidic flavor for the summoner?
Pull in a few Druid spells, would be the most obvious way. Stuff that's mostly Conjuration-oriented, like
Fire Seeds- though there aren't a lot of Druid Conjuration spells, so perhaps extend it to non-Conjuration stuff that's obviously Druidish such as
Entangle and
Goodberry. Maybe grant a couple of minor Druid abilities that make some sense, like Woodland Stride, instead (or in addition).
Alternately, you could improve the Archetype by going a different way: really play up the Fey aspect of it by making the Summoner himself/herself more Fey-like. Again, I think changing the spell list is the best place to do this; add some Enchantments to the list and you've pretty much got it- I'm thinking the
Charm suite,
Confusion, and maybe some Illusions as well like
Rainbow Pattern. Also grant the Summoner an extra bonus on Will saves against such effects, and/or against the Confused condition since Fey are closely associated with (causing mortal) insanity in folklore.
Summoner is a pretty powerful class in the standard form, so even a nerfed one can be pretty strong. If you want to make the Archetype "viable," you shouldn't have to do major changes- just a few minor tweaks. Additions to the spell list are an especially good way to pull this off because the Summoner has limited spell knowledge- sure, there are always Wands and Scrolls, but face it, there's also Use Magic Device for those in the standard Summoner's case (and it's even a class skill). So adding a few spells to the list isn't a large power boost, but you can really change the flavor with some carefully-chosen additions.
