I have a couple of questions about what's possible with OpenXcom, so I'll list all of them here.
I believe some of this could be managed. I know base tiles are fixed at 6x6 BUT if you're talking about the Map Tiles they take up during a base attack you could technically retrofit all of the base map chunks to be whatever size you want... instead of 10x10 per facility they could be 15x15 or 20x20 per facility (with a 2x2 facility being a monstrous 40x40 map chunk).
2) The Access Lift can be transformed into a different kind of facility such as a combojunk thing so that every base comes with a standard assortment of stuff. It could even be a 2x2 size facility (or even 3x3 size if you want to be awful). You get an "Access Lift" facility for free and each base requires one, but as to what an Access Lift actually is in reality is up to you...
(it could be a 2x2 combo suite that has radar capabilities, a barracks, storage, and a small holding pen for aliens, along with an access ramp, all wrapped up in one package.)I wonder if you can create an additional base facility that functions as an access lift, thus allowing you to be able to sell/dismantle the actual starting access lift, and replace it with a cheaper or more functional version later on?
3) Yeah that would be really useful for having a Small Craft launcher sorta like an ICBM silo, which would be a nifty game balancer because you can say "nope this craft requires a special hangar for it and it wont fit in the standard hangar". Also for launching drones of various kinds that don't require pilots, while your hangar craft require pilots.
What would also be pretty nifty is being able to TURN OFF the appearance of craft in a Hangar so that you cant see what's actually docked in there. That would make multi-capacity hangars more feasible because then they wont be stacking sprites on each other. Likewise you could also make Hangars a 1x1 size facility (regardless of the gameplay balance issues) since you dont care about the craft sprite appearing there.
(going off the idea here of resizing the inherent base map tiles to be larger than 10x10 you could retrofit most facilities to have a short corridor linking between each of them, but facilities meant to be larger such as a hangar would take up most of the 15x15 map tiles. Then for a truly huge facility such as a "Factory" you have 2x2 and it ends up being 30x30 map tiles)
