Aye but you want game-play customization then you run the actual titan list or take one of the sub-faction lists that provide certain additional components/options.
So first of all, as already said above, providing any semblance of balance and testing has an inverse relationship with the number of permutations on the list. Hence why many lists have very structured design around a theme.
Second, lists, by design, have inbuilt weaknesses that need to not be easily compensated for by a giant walking machine of death (or actually any unit / formation whatsoever), hence why units/weapons are restricted.
Third, your weapons on your model really have very little limitation on what you play on the table. Let me give an example here: The Codex Marine list has a single Warlord loadout. Regardless of what Warlord you drop on the table with whatever weapons you have, I know what it does. I don't care that it has rubble claws and a wreaking ball glued on as it doesn't change the game-play. I'd look at that and go, "awesome titan, dude!"
Fourth, there's lists that specifically cater to titan-heads that want that extra flexibility (and in return have more restrictive structures/nerfs to compensate). So for full on titan craziness, that's AMTL but you could also play something like Iron Hands and Imperial Fists which have different Titan configurations that fit their theme.
Lastly, the NetEA lists are there for general tournament play. Nothing stops you from taking the structure from say, AMTL for costing and apply that to your nummy titan-of-stompy-doom (ToSD from now on
) and use that in friendly play in say the Steel Legion list.
Hopefully that clears some angst up for you, mate.
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