I really love how flexible he is especially in terms of builds. He is one of those champs that can build practically anything and make it work, even though I prefer to play him more dmg oriented.
This is a build I used to run in the past and still do occasionally when I feel like I will be on my own in this game.
full-Ap-OMG-I AM MELTING AWAY IN LIANDRYS-MEEP-BARRAGE-BUILD:
+
/
/ Defensive item
It´s probably the best solo carry build and really shines in teamfights (kind of like an ap twitch who is not complete bs), but you will note that there is barely any cdr and that you are much slower without mobis and ms items so you have a few restrictions to work around. You can´t tunnel all the time, you are more reliant on good ults to engage, and everyone on the enemy team will start to focus you instead of the carries which is hilarious and sometimes outright wins you some games.
Then I tried to incorporate athens into my builds so that my heals wouldn´t suck anymore and to get value out of athene´s second passive I added more and more heal items to it, which then ended up becoming a completely different build.
the Ap-ambulance-build

+ stone born pact or however it´s called now. The thing that heals allies when you slow enemies or in bard´s case when you AA somebody.
Healing may be its main function, but it can still run you over if you are not careful.
And right now I am tinkering more with early cheese and really fast meep acquisition, which requires a lot of movement speed and as we all know only ludicrous speed can suffice here.
the [I am here, I am there, I am everywhere]-build

+ relentless hunter
And ofc there is the good old on hit build which is quite funny.
The one thing I would never do is building him as a tank. I see a lot of players building
and even though they can be annoying, they never have as much of an impact as they could have had.
In the end you are ranged, you even have additional reach with your meeps, you don´t profit much from being in the center of a fight like an alistar, leone, or taric would and you don´t get bonus resistances. So I would just focus on either more utility and or dmg.
These bulds may not be equally effective, but they are all fun to mess around with.
And whatever riot does with him in the future, I hope they don´t cut in his dmg output. I really like how he scales with meeps and you kind of have to be able to bully the lane to even get the chance to roam.