I understand that this idea can be applied only in very far future(maybe version 2.4.65, or later)) , but how about convertor from ascii maps to original DooM ][ ][ maps? So if you extend quantity of DooMRL levels to 30 + 2 secret, they can be converted directly to new DooM original wad, and be used as new episode. As i know, level modding of DooM is not illegal.
And what about adding really secret level(s) on single map(on standart random level, or on special level)? They must be hidden (you'll need to create some rooms that are not easy to pass)
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#..#>#..#
#..###..#
#.......#
#.......+
#.......#
#&..&..&#
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Levers must be pressed in the right order(if it is possible to programme on this step), otherwise some powerfull mosters summons, and floor floods with lava(maybe without lava will be better). There can be more levers to harder switch-picker work, and there must not be able to see with Intuition, what every lever are doing.
Or maybe place stairs to secret level on Hell Caves level somewhere deep in the rock, and the only way to make it to them is to find little passage in hard-reaching place. Of course no computer map must be spawned on this level.
There can be 2 or 3 secret levels every time you play(exact secret level is chosen randomly from some precreated maps).