Ever since the jump to version 1.16.5, I have been considering the idea of Nether-only playthroughs. If you were to attempt a Nether-only playthrough of Pixelmon, the main challenge in your journey would be obtaining a PC, as it is even more difficult in the Nether than it is in the Overworld. The only two methods of obtaining PCs in The Nether that I know of are time consuming and hard to pull off. The first present method is to craft several Healers using diamonds obtained from chests or Sableye as well as iron and aluminum obtained from Pawniard or Honedge in order to obtain Porygon parts. Porygon has a 10% chance of dropping a PC when defeated. However, this method requires wasting diamonds to craft more healers than a player will need until a Porygon finally drops a PC. When I crafted a whole stack of healers, I only obtained enough Porygon parts to craft 4 Porygon. The second method is to defeat several Dunsparce, which is a very rare underground spawn and has a 50% chance to drop sand. However, Dunsparce is so rare in the Nether that I have never seen one there despite spending a lot of time in The Nether. On average, the player would need to defeat 12 Dunsparce to have enough sand to obtain glass panes. The Porygon method is probably more reliable than the Dunsparce method when trying to get a PC, but the Porygon method involves a lot of wasted resources and the Dunsparce method is the only way to get a Trade Machine if one is desired.
Obtaining a PC is probably the hardest part of a Nether playthrough of Pixelmon. Apricorns can be obtained from foraging, dirt to plant them on can be dropped from the Drilbur line or Diglett line, and iron can be obtained from the Pawniard lines and Honedge lines, so Poke Balls are farmable. Wood tools, stone tools, crafting benches, and furnaces are obtainable by the same methods as vanilla Minecraft. Healers are obtainable by the method explained in the previous paragraph. Shulker boxes are obtainable due to Turtonator dropping shulker shells and chests being obtainable from crimson planks or warped planks. The main other challenge I could think of is obtaining food, though mushroom soup is still craftable just like vanilla, golden carrots are found in chests, the Salandit line drop Shuca berries which can be eaten, and the Tepig line found in limited numbers in bastion remnants drop porkchops.
Lampent can be reliably found in The Nether due to it spawning from Blaze spawners. Therefore, it is a good candidate to drop glass or glass panes to make PCs more obtainable. As a bonus, no Pokemon currently drop glass or glass panes according to the wiki, so Lampent would have a unique drop.
If you don't want Lampent to drop glass panes, then another method to make PCs more obtainable is to approve my suggestion for more Better Spawner categories for the Nether, as some of the Pokemon that I suggested for the categories (namely the Skorupi line, Darumaka line, Sandile line, and Sandygast line) currently drop sand, which can be smelted into glass and then crafted into glass panes.