What i think of with Tank Assassins

You can send Ekko to top lane... or jungle... and he can often do so quite well. Even when sent to mid he can lane clear just fine bar a few match ups where he can still wave clear he just may miss some CS. He has a decently strong ranged attack with Q for clearing and harass. He has his Q + his W both of which can be used, each on their own, to allow him to move forward long enough to clear some creeps if they aren't under his tower. As a champion, he is extremely safe due to his ult, CC, shield, dash, and range/zone control even if he isn't fully ranged like many other mids. Yes, he has less kill potential early on against some champions depending on the match up if you don't use his ult properly and are going tanky at the start. It is not fair to compare single target assassin damage of Zed/Leblanc to his strong AoE damage and solid single target.

Yes, his W is telegraphed... and he also has a huge amount of zone control to force many champs to get hit by it. If they have the ability to evade it that means they likely aren't killing you when running from it. If they try to all in on you most champs will be hit by it. You can focus on not going for in-lane kills if it isn't a solid match up and focus on feeding kills to your other lanes/jungler. This is a very basic tactic to be fair.

He has a shield and quite a bit of CC/ult heal and mobility to avoid a lot of harass and get out pretty fast. This complaint can't be valid for Ekko yet so invalid for Fiora. With Vicktor he can easily avoid his W/E with his dash and his zone stun is far more frightening than Vicktors while having virtually the same limitation... Ahri can be side stepped for two of her abilities and Ekko has more than enough mobility to do so, especially when far less mobile champs can do so reasonably well.

Don't get grasp if you aren't going tanky Ekko. Many champs top and don't have sustain or strong sustain... and Ekko has his Q/shield and ult's heal as well as mobility/escape potential making him a lot safer top than many other options.

Malphite and Rammus are both melee and particularly susceptible to Ekko's Q/W. Cowl is particularly good against short trades. Against Malphite you could take Cowl as well or more Mres runes/masteries. This doubles up in Ekko's favor as Ekko is superior at extended trades and zone control to Malphite. In an extended trade Malphite does not benefit from his passive as much, or Cowl, and he has no proper method of trading hits after his initial CDs pop while Ekko does. Ekko can also juke his E with his own E and then move back in with the blink portion. Ekko can force Malphite to choose between sustained trade for CS/brawling or getting his passive back up to trade again at the cost of CS. Rammus you can simply not engage when he curls and Ekko definitely has the tools for this. Yes, some bad match ups may exist but he can be played safely so that he at least gets CS, maybe a kill or few if jungler can help, and assist other lanes/jungler or bide time until mid game. Heck, any champion with semi-decent ranged wave clear can usually survive a bad match up without falling significantly behind. Ekko is both safe and has an alright wave clear potential. If it is a particularly bad match up see about swapping lanes. In draft modes this is less of an issue and easier to compensate for.

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Ahem

and to a point

Let me bring Renekton mid. He'd get wrecked there. That means his laning phase is weak, right?

No, it doesn't. Also, one of their bases could be modified to not be excessively weak with those changes as well. Kat and Akali primarily use one skill during laning, while the others are used to help with last hitting for the most part. This makes them very safe in the fact that they have a ranged last hitting tool, as well as minor escapes/disengage. Being melee doesn't mean your laning sucks. Its just that people bring them mid against ranged champs/champs that are broken. Kat and Akali specifically have been nerfed to the ground for what they do, and with a minor rework would actually have pretty strong laning before these changes I think assassins should get.

All that needs to change is the all-in damage early and scaling so assassins require items before they can deal enough damage to be aggressive solo (which would also mean they couldn't effectively build tanky anymore). Keeping their farming tools usable, or even more usable, allows them to have a similar pattern to someone like Irelia back when her early game was extremely weak, but her laning was strong overall.

It also gives assassins a clear goal: play safe early so you can hit your mid-game power spike and wreck glass cannons. This is both a consistent gameplay element, and a satisfying one. It doesn't rely on snowballing like now, it relies on playing safe in lane. The disadvantage is that you almost will not get advantages early; you will be playing catch-up, as well as the chance that they could coordinate dives against you.

don't forget that while & may be mobile and damaging early, they both have awful waveclear. zed has both.