So I was looking over the changes for the colossus next patch, and I think necro tank will be able to keep colossus uptime around 90% without having another necro in group.
3 ulti/sec from light attacks/blocking
1.5 ulti/sec from minor heroism
0.5 ulti/sec from nord passive
6 ulti/sec from necrotic potency
0.625 ulti/sec from corpse consumption passive
2.6 ulti/sec from bloodspawn
If you can keep everything up, that's
14.225 ulti/sec total
Colossus costs 225 ultimate.
With expunge it costs 218.25.
The debuff will last 15 seconds according to PTS.
218.25 ult / 14.225 ult/sec = 15.3 seconds to get colossus.
So in an ideal world that's 98% uptime on colossus from one necro in group. That won't actually happen in content, but and 85-90% uptime seems very doable.
Another thing to think about is alkosh leaving the meta though. That leaves a free set for tank to run, so if the necro tanks replaces alkosh with an ulti set, they could definitely achieve 100% uptime on colossus.
Example 1: arkasis
44 ulti /45 sec cooldown on potion = 0.97 ulti/sec
Add this to our regular ultigen with bloodspawn 14.225 + 0.97 = 15.2 ulti/sec
218.25 ult / 15.2 ult/sec = 14.4 seconds
So with arkasis in ideal conditions, a necro tank can keep colossus 100% uptime. By adding in potion cooldown jewelry this can be made more reliable as well.
Example 2: akaviri dragonguard
15% ult cost reduction means colossus costs 218.25*0.85 = 186 ult
186 ult / 14.225 ult/sec = 13 sec to colossus.
Dragonguard makes it a cake walk to keep 100% uptime on colossus with only 1 necro. The downside is that it's more selfish than arkasis which gives 3 group member ultimate as well.