Potato guy at Frogatto home - he could each time You uncover new level or portal pad have something new to say, some story or useful tips.
Shop - rise price for different stuff - usually I visit it only 2-3 times during game - first when I reach it on my way to town and I have every coin and next time when I have enough coins to buy rest stuff. It could be better if some items appear at shop later.
Monster book at Frogatto home - an item that act like a monster list, each time You kill something new there will appearer record about it. Something like that was in Castlevania.
More optional challenge levels with gold statues as reward just like those two from castle, those gold statues can appear at froggato house as decorations when unlocked. Also arcade games could be access from that place too - buy them at shop and play them at home. Also - when You get achievement - one small star will appear at froggato house as hanging decoration. Unlock them more and frog guy will have nice stars map on his celling.
More tasks from Nene to do 
More different house indoor design right now every house got same design inside, think about different colours for walls.