Speaking from City's pre-season training base in Sweden, Millen told BBC Radio Bristol: "There's plenty more to come from Cole.
"He's been made skipper a couple of times out here and the manager [Steve Coppell] likes what he sees."
Millen added that the coming Championship season could be significant for Skuse.
"He has every attribute now to keep improving. He has the mentality and works hard so hopefully he can have another great season and help us push for promotion.
"He's played a lot of games for us and he's not a 19 or 20-year-old any more, so has that experience to his game."
Millen added that the Swedish trip had provided useful training and match practice opportunities.
City drew 1-1 with Gothenburg and lost 4-0 against Helsingborgs in the first two of their three games.
"It's a case of getting the squad as much match practice as possible," Millen continued. "We're still looking at different formations and partnerships around the pitch.
"The manager wants to see them all so we're chopping and changing at the moment.
"We've been able to do some good sessions in between the games. The more time we can spend with the squad the better."
Source: BBC Sport
Source: BBC Sport