@Double
For a Horus deck, it looks like you're forgoing a lot of its support cards, if those interest you at all. As for a new custom card... Maybe a special card?
- Since there's only one custom card per player, a Ritual is out.
- Fusion would be a little weird for a deck based around a single monster leveling up.
- Synchro would need a tuner, but since lvl 10+ is god territory anyway, maybe a level 10 synchro? I'll come back to this in a moment.
- XYZ would need multiple level 8s, which without going through your whole deck list, I don't know how plausible that is.
- Pendulum is useless in a deck with only one pendulum monster.
- Link maybe? A link 3 or 4 Horus that requires Horus Lvl 8 to be one of its chain links? That's a little more complex to come up with than the other kinds, though.
Maybe something like:
Then it's just throw in Majestic Dragon, and 1+ Tuners such as Rider of the Storm Winds to buff out the possibility of sychroing with lower than level 8 Horus.
It's not an actual Horus deck. It's just a dragon structure deck that has Horus splashed into it. That's why it lacks the LV support cards.