There are many posts about upgradable TCs. Lots of good concepts but some flaws as well.
Having a tc that can be upgraded armor-wise would be idiotic in some instances. If you knew the base was a 16 c4 raid to tc only to realize that the tc is another 2 or 4 or 8 or whatever would be frustarating and unnesscary.
I like the idea of a way to anti-grief without multi-tc but most people just need the stuff inside of the tc.
This idea could be easily countered by not allowing upgraded tcs to have locks, so that they can be looted without being destroyed.
Another suggestion is to have upgraded tcs have more slots, which is fine. But some suggest less upkeep time for more upgraded TCs. This is not good either because zergs with seemingly- unraidable bases will just be even more unraidable