D&D 5E Ten Years Anniversary OneShot for Evil Guys

[RobAndry]

Hi Folks! Here i am for an advise request about a Ten Years Anniversary D&D OneShot!
Yes, ten years have passed since the last game I made with my high school friends and now I'm planning to organize a OneShot(maybe TwoShot) with the same Characters of that time.
The Character group of evil guys were:

1. A noble ExPaladin/Black Guard that was in search of the right and bloodly glory for his Family
2. The brother of the Black Guard, a Wizard/necromancer on the way of becoming a litch
3. An Hextor WarCleric in serch of Battle and Bloodbath
4. A Sorceror/DemonLike obsessed by Power
5. A Crazy Gnome Barbarian that follow the whisperings of a speaking gem in his possesion.

The main plot of the campaign was focused on the reserch of personal power that leaded the PCs at the orders of a powerfull lich. The PCs obey to the litch for a while increasing theyr power until the litch betrayd them. In the final battle they managed to beat the litch and earn their freedom from a dimensional prison.

I'm in search of hint and advises for planning a short game with this Characters.
For now I'm breingstorming for good ideas, with nothing clear in my mind, but i have some key point, the game MUST have:
- Max Duration 8 Hours(one/two game session)

- Must have one Great Battle

- Must have one part based only in role playing

- I'd like to introduce something outside the box or with a crazy style.

We used to play D&D 3.5 edition, but this in unimportant, because for now I'm want to focus on plot and inspiration.

I am waiting for your advices!