1) That would be a lot of work ; actually there is no animation set at all for riding something. And I don't even know if it's possible to add some. You can add particle effects and other things like that, but adding a character animation is a lot more difficult, as you have to parameter every moves (the hands, the arms, the legs, and so on), and I don't think GUTS has a layout to do that. But maybe it's possible, who knows ?
2) Hmm the soulstones will be like that if I have correctly understood. But, i would be ok for a kind of special sockets that you can whatever you want with it : put a rune, or take a rune out of it, without breaking the equipment. And the rune would have different effects from the gems ; if that was some kind of "you get that, but you get this penalty", i think it would add something to the game. For example "1% to cast meteor strike on Target on hit. But : 10% to cast meteor strike on YOU when you get hit". Things like that would be awesome
3) Hmm.. What do you want to do with wings ?