Ok, I hope to see ideas like that one day (although I suppose it will be very difficult and tedious to program).
Regarding the sewer mission, I don't mean one thing or the other. Let's see if I can explain myself with google.
As a player I perceive that this mod offers a perspective of a more detective x-com. That is, not only is there to stop the invasion, but to solve all kinds of mysteries. In that sense, the mission of the sewers would not be out of place (and personally I don't find it boring either).
The problem is that I see that the mod has an "internal logic". Threats fall into two categories:
1º Those that are part of popular folklore (they are myths and legends about creatures and events that do not appear in the mod)
2ºThose that in the end do turn out to be real.
Everything that is part of the second group becomes a priority for x-com, either destroying the threat and investigating where it comes from (and already reaching the conclusion if it is better to "leave it alone", as if it happens with the dimension x, or try to destroy it permanently).
It is true that not everything has to have an explanation (in fact, sometimes it is for the best), but there is no investigation after the mission that tries to reflect that x-com has tried to investigate the nature of the threat (or at least is very vague, treated more like a legend than something real).
The "wizard did it" philosophy could also be applied, but I think it would then work better as a self-contained quest, like the thing's quest, that had a reward upon completion (eg receiving 1000 points).
In conclusion: it does not add anything to the story, it does not pose any threat (as it has a very low penalty and there are many other missions to do each month), it does not have any reward, and I feel that it does not follow an internal logic either. Since there are many ideas to develop, it is still better to discard this idea (or at least reform it, even if it is not in the form of an arc).
Thanks for the patience!