I think the idea of a "time thief", as in someone who uses time travel to steal things, not someone who steals time directly, has some really cool thematic options.
... Actually, someone who steals time directly would be cool too.
Unfortunately, the standard tropes for a time thief overlap with Ekko REALLY hard, so there would need to be a lot of work to figure out some alternative design that appeals to people wanting "thief" and still making sense with time travel.
Other than that, I think actually stealing an item would actually provide very interesting gameplay potential, though it seems difficult to make it intuitive or seamless. If you use some classic thief concepts, the champion would send a "calling card" ahead of time indicating which item will be stolen. In fact, it would be far from seamless but that could at least be a thematic way to implement the selection interface: You use the ability (almost certainly the ult) and it brings up a menu that looks like a card, with something like "Dear _____; I'm coming to steal your precious ______." (or something with actual flair) and the blanks are dropdown menus so you select a champion and then choose an item from the last time you saw their inventory. Clunky sure, but I imagine that any item you would actually want to steal is one that wouldn't be likely to suddenly LEAVE their inventory for other reasons.
After you send the calling card, there would be a significant delay before you are actually able to steal it (sending a calling card and not allowing time to prepare would be unsportsmanlike!), as well as a maximum duration so the target can rest easy if they do successfully guard their precious blank. In my mind a successful theft would only be possible within melee range, and dying would instantly return the item so you better have an escape plan.
After a successful theft, the target would no longer gain any of the effects of the item stolen, and the thief would gain them instead. In order to make things work out, I would have the item "stay" in the victim's inventory but greyed out to show it's absent and unusable, while for the thief it would show up in the R slot and pressing R again would use the active portion (if applicable). Of course, after a while the item would be returned to its original owner, as a gentleman should return what he borrows (even without permission).
Thematically, I'm picturing an elderly Demacian noble with a British accent, wearing a tuxedo, top hat, cape, and a mask to conceal his identity. Probably a cane, in case he needs to tell someone to "have at you!" By day he does unbearably boring noble stuff like squabbling over political position, but by night he goes out to secretly have a jolly good time making everybody else look downright foolish. Ever the showman, he's improved a long-standing love for prestidigitation into a capable and flashy tool for absconding with priceless trinkets, scandalous secrets, and the hearts of maidens who are into that sort of thing.
The only basic ability I can think of is a classic trick reminiscent of Lissandra's... claw thing (who has time to look up ability names/keys when you have dashing british magician/thief characters to describe?!?). You throw your hat, and early in the ability you either reactivate it or don't. If you reactivated, then you appear with a flourish at the hat's location. If you didn't reactivate, then it turns out the hat was an illusion! It transforms into a bouquet of roses and the magician becomes invisible while everyone's distracted! The mind games would be fun, and it's a great tool to get into range during an ult - as long as you correctly choose which version of the trick you need.
Another ability that I'm less confident in is, making projectiles disappear. Flourish the cape, and all those pesky skillshots vanish mysteriously! For a second trick, make them reappear - going the other way of course. Actually, it seemed difficult to make it both balanced and rewarding, but how about if it can only do a single skillshot projectile (no targeted anything, some kind of tiebreaker if multiple skillshots are within the range) in a very small area in front of him, then reactivate to send it back out with the same damage but able to hit the enemy team. Actually, rather than have it be "A skillshot in this area", it should be "if a skillshot hits him while the ability is active", like Fiora's new parry except a bit different. That way, he'll need to be positioned directly in the way of the skillshot for it to work making it very punishing if he times wrong, or if he fails a prediction.
Third basic ability should probably deal damage since he doesn't have any direct damage elsewhere. Maybe just something simple since he's fairly complex elsewhere, but (imo) something with both AD and AP scaling in reasonable amounts so that he feels good about the stats regardless of which item he steals, and range since I'm picturing him with melee basic attacks. No CC, that would be too much. That's actually most of the ability designed, it just needs numbers (like everything else) and visuals that match the magician/thief theme.
Passive is a bit tricky, but I did have one idea. Pretty simple (but probably crazy OP), he gets 1 gold any time a champion CSs near him... on EITHER team. He's a noble, gotta have taxes. Of course, they wouldn't be the kind of taxes that take away from what everybody else gets (that would just be stupid broken and unfun) but more like... Hypothetically, every minion in the game is worth 1 more gold than is ever indicated in game, and that gold is taken as taxes by some sort of organization... perhaps it's just a Legend, but it would be in League with all the governments of Runeterra to distribute those taxes through some indecipherable mess of tax code. It turns out, somewhere in there it says "if this particular noble is next to that minion, he gets those taxes" and he's only avoided discovery because seriously NOBODY is going to just go reading through random tax code. Jokes aside, It might be hard to balance and fairly boring, so I would be happy to see a better option even though I love my description of how it works. Heck, it could be referenced somewhere in his lore that "He ostensibly still participates in political jockying, but is actually set for life due to working several kickbacks into the tax code that are too small to notice individually, but add up quite significantly."
So... I didn't actually have all of this in mind when I started writing the post (it was actually supposed to be an afterthought for the time thief thing), but it turned into like... 60% of a support's kit. And as a support main OH MY GOD RIOT PLS. I will swear that I never posted anything if it means you are allowed to steal my idea and put this into the game I will main him so hard Braum is cool and all but I want to be the dapper rogue that one-shots a fed enemy with their own ability
ultimate steals AR/MR/HP, and his Q steals AD.
Q steals MS.
