I was considering doing a quick and dirty mod to Meld into Eidolon to make it worthwhile, and letting a player use their mental ability scores while in that form. They're not a power gamer, so they're not going to get up to shenanigans.
What have you done, if anything?
Meld into Eidolon I think is an attempt to achieve what could only be done properly with a synthesist class archetype, but if you want to make the feat more worthwhile, you could allow the eidolon to cast your spells using your statistics and spell slots + cantrips while melded. You wouldn't be the strongest still, due to the loss of tandem actions, but would at least get to draw from both your Summoner and eidolon's abilities.