For the last two skins I would love a marauder skin for Talon and a warden skin for Diana. I feel the skins would work well with these champs. Talon is the definition of dark and merciless. A marauder skin that plays off the fact that he is not a armored tank, but a quiet merciless killer that seeks his own personal gain would be awesome. Talon hasn't received a descent new skin in a while. The SSW skin was okay, but I didn't feel unique and it is a legacy skin which means it won't be on the store for a while.
Diana would work perfectly with the warden skin. She is a warrior of the moon. She thinks of herself as above the law. Her abilities would feel adequate with the skin since her abilities have light colors and embody the idea of Diana being a transcendent warrior. Diana only has two other skins and only one of them feels adequate with her personality and mannerisms. A warden skin would perfectly capture what Diana is as a champ.
As for your opinion, I think Jayce is a interesting idea for a warden skin, but my problem with it is that he is not a mid laner. He is more of a top laner. He is also very popular which means Riot will shower him with awesome skins in the future. I wouldn't be mad if they gave him the last warden skin though. I feel that a warden skin would fit him well.
As for Twisted Fate, he already has a lot of skins. Giving him a warden skin would be somewhat pointless since he has other skins that are way more popular. His personality and mannerisms don't seem to resemble that of a vicious marauder. Twisted Fate would make an interesting warden though. If Riot decides to continue to the wardens and marauders skins (which they should), I would like to see Jayce and Twisted Fate get a skin, but I feel the priority for this possible last skin to be for champs that have been somewhat neglected in the skins category.