Well, you might have to wait until Infinity comes out for an in situ editor, or one that can handle sprites, but this nifty little tool should help immensely in the meantime. It allows you to extract wall textures from M2 Shapes files, and save them in M1 format. You can then edit them with any of the Marathon 1 Shapes editors, and then reinsert them. I only did preliminary testing, and there are quite a few restrictions (read the docs), but it seems to do exactly what it say it'll do. Texture editors, go at it!

Thunderdome with a large pool in the center. I'm not sure if it still has a hill... The arrows in the side rooms are fun.

A nice wheel-like ring, but you can shoot through the inside wall of the outer ring across the center. Made for KOH, but be careful... the Hill is surrounded by polys that'll teleport you into a bob-crusher you can't escape from. One good grenade at someone standing on the hill is usually enough to knock them into the bob-crusher circle, if they don't start moving immediately. Update is no longer 68K-clean, but has some lighting updates.

Claustrophobic. One big room, a bunch of too-small ones... and lots of ways to blow yourself up. Jarring textures, as well... Should appeal to the Carnage Palace Deee-luxe crowd.