League of Legends (LoL) Question: Champion Pool

Mordekaiser and Sett so you can flex top as well. Vel'Koz, Xerath mid/support. Ekko mid/jungle. Expanding champion pool should be, in my opinion, across roles not just one lane.

hello! as said about the meta, I reccomend staying with apps or searching online for the meta and following updates.
when I look to try new champs , I like to search and see other people playing that champ. it will help you in figuring out if you like the playstyle this champ offers a little better. (im really not sure how to link a champ, so sorry about that lmao) but about recommending you champions , i would recommend ahri, syndra, diana , lux or zoe. i think ahri lux and zoe are fairly easy to learn. syndra as well. diana kinda remind me of more of an assasin gameplay- shes nice to learn as well. i would direct you to ekko as well - he doesnt count as mage in the sort by role category in league , but hes really fun as well
( sorry about any grammer mistakes! not a native speaker but trying my very best!) hope i helped a little. :)

If you want champions that are good in the meta I'd recommend you check an app/page with a regularly updated tierlist, otherwise any recommendation you get may be outdated without you knowing. Personally, I use op.gg/champion. You have a tierlist to the right where you can order champions by position, tierlist and/or winrate.

As said by Prate_k, it's good practice to try and expand your pool including champions from other lanes. That would allow you to better understand your teammates and threats in other lanes. That said, you can do both by learning champions that can be flexed in different lanes.

Right now, Ekko, Galio and Viktor seem like good options for you. Ekko is kind of a middle ground between mage and assassin (you may already play him) and you can (sometimes) play him jungle, depending on the meta. Galio is a great counterpick against most AP champions and can be played support. Lastly, Viktor is a great pure mage who for some reason is also being played top, crushing a lot of champions there. All of them are also evergreen picks that always manage to stay playable in any meta.

I personally play toplane or ADC. For toplane I mostly play Shen, Kayle (S tier picks) and Ornn (A tier pick). As for ADC I play Jhin but also good options are Miss Fortune, Caitlyn (those 2 are A tier) or Samira (S tier as Jhin)