I want to leave a little bit of feedback.
I tried Lissandra with the Colossus mastery and it felt absolutely amazing to play with her like this. Going in balls deep while not fearing for your life and immediately needing to ult yourself or use zhonyas.
He's my idea, which I think might work since she has both soft and hard cc in her kit: using hard CC she gets a shield that scales strongly per level and weakly with AP (no hp scaling). She gains a shield based on the amount of enemies she hard CC's. Every time she CC's an enemy in general, the CD of the passive drops (cd also heavily drops with levels, starting with an incredibly long cd, but ending with a shield you can proc twice in slightly longer teamfight if you cc people). It also drops per person hit by the cc. Half a second for one person hit with a Q, one second for two people hit with one Q. Something like that.
Another idea, which is more reliable, less 'colossussy' is still a shield, but one on a charge system. Same with the other shield, it has a ridiculously long cooldown at first that gets better rapidly (think malz' p, only with cd reduction every level instead of every 6 levels) and starts with a potential 2 stacks at level one to 5 stacks at level 16. Every time you hard CC an enemy, you consume a stack and gain a shield, and this happens for every enemy cc'd. There is no cooldown in acquiring shields, however, if you cc the same person that you still have a shield from again (like a W and R combo), it refreshes the shield. Shields are only additive if you hard cc unique champions. Once again, CC (hard and soft) lowers the CD on the charges. I think this one is a bit more healthy, workable and unique. It gives healthier trades (less predictable in the laning phase than a general shield because of the time in between stacks consumed instead of a static period of terror, while also not suddenly making her completely defenseless for half a minute after a trade in the laning phase. This is okay because Lissandra's E is very slow so she's already rather predictable)
Also, the shields should be very small as one, but big when multiple stacks at the same time are consumed. This is healthy for both Liss and the enemy.
Another problem this solves is that it doesn't suddenly make Lissandra a backline diver that can't be killed because of the shield but still blows up your enemy: Lissandra heavily relies on hitting enough enemy with her CC to get a shield big enough that doesn't make her die instantly while trying to blow up the ADC (the E is slow and anticipated). Instead, it makes Lissandra want to be in between the front and backline of the enemy.
And the reason acquiring a shield doesn't need a cooldown is because she only has one ability that snares a lot of people at the same time. Until that one gets back up, all she can get is a small shield from her R. Her W cd is essentially her shield cd.
Either way, I think the 'charge system shields' system is interesting, even if it isn't for lissandra. Then again, she basically has the perfect kit for this very idea.
Also, I know you tested 'liss with shield passive' before and it didn't feel good somehow, and I assume it was because it made her basically Diana (full damage, blowing up the ADC and getting away with it). My idea is a bit more nuanced that favors teamplay way over assassin potential. Also, I think it's easier to balance since a shield is very early too big for 1v1 but too small for a teamfight.
And finally, about colossus: Can we make (de)buffs pingable? When I play tank and my colossus is down, I want to be able to ping that, and how long it takes until it's up. 30 sec is a long time to estimate for your teammates. I also think it's a good idea to be able to ping the baron buff, or a passive to an active ability that's active, or the amount of stacks you have as nasus, veigar or thresh. Sometimes I feel it's also useful to ping that you have grievous wounds so that your support should wait with healing you after a trade.
I really hope you will consider this idea for testing.