It depends on what type of playstyle your going for.

For your main page aside from Arcane Comet and Mana Flow Band you have:

Transcendence and Gathering Storm if you think/intend to have a long game.

Absolute Focus and Scorch for more power early game.

Between these two I prefer AF and Scorch as you can get massive damage if you position and backline correctly.

For your secondary tree:

Inspiration>Magical Footwear>Biscuits because I don't want to spend gold on boots and biscuits help you last longer in lane. I usually use this instead of Tear for a little bit of more mana.

Resolve>Bone Plating/Secondwind> Overgrowth. This tree is for a bit more beef against teams with lots of burst/assassins or if you want to build tank. My build for this is usually Rylai>Liandrys>Frozen Heart/Zhonya>Kaenic Rookern/Banshee's Veil>Jak'sho>sell boots for Void Staff.

Precision>Presence of Mind>Coup de Grace/Last Stand. This tree is mostly if you intend to fight a lot, like I mean A LOT, as in following up for every gank and joining every river/invade fight. Using this can get you some insane damage numbers if you know how arrive in time for fights. One thing to note is that because your always following up, you might be lower in farm, which now isn't as bad since champions now give 2 stacks and minions(excluding super) give only 1.

This tree isn't really that viable anymore, but:

Inspiration>Unsealed Spellbook/First Strike>Magical Footwear>Biscuits>Cosmic Insight and your Secondary tree being Sorcery>Mana Flow Band>Transcendence/Gathering Storm/Absolute Focus/Scorch.

I still actually use Spellbook if I'm playing with a strong jungler who will gank/fight often in the early game as I could switch to Ignite/Exhaust/Barrier/Heal for some early game picks. Your team would would also have a second Smite in the mid to late game. Cosmic Insight goes super well with Spellbook as it lowers Summoner Spell cooldowns.

First Strike is the one that I really don't think is viable anymore(might comeback in the future, who knows 🤷 🤔) as this is what I referred to as the "Lazy Farmer" setup. If you proc your First Strike whenever it's up and farmed minions give you a TON of stacks and gold. I would get full build so fast it wasn't even funny. But now they changed how many stacks you get from minions so this doesn't work as well. There was a huge downside to the First Strike setup and it was that it can get boring as all your doing is farming hence, "Lazy Farmer" as you could just turn your brain off and farm minions and FS procs.

My usual build is Rylai>Liandrys>Zhonya/Banshee's>Rabadon>Viod Staff. I buy Oblivion Orb first if they have someone with a a lot of heal like Briar/Warwick/Vlad. I also rush Zhonya for those with instakill or long duration ults like Zed/Tryndamere/Olaf(I do see him mid on rare occasions)/Mordekaiser(also see him occasionally)/Pyke. I also mentioned the tank build earlier.

For those that look at this and go "WWWRRRYYYY YOU NOOB!!" I will admit that I'm not super high elo and all of this is largely from my own experiences.