Don't act like we'll get much more than him randomly appearing in STAR Labs, giving exposition and then waking out.
How he's going to randomly appear in a time ship or another universe is a little tougher, but I'm sure it will happen.
I mean, STAR Labs and the Arrow Cave both have Cisco's security. The DEO isn't much better.
Barrowman stayed great. What was infuriating about Malcolm was how much everyone else became an idiot in order to justify keeping him around. He murdered hundreds of Glades residents, killed Robert Queen, drugged Thea into killing Sarah, and set off a war between Team Arrow and the League of Assassins that almost wiped out Star City to save his own ass. But sure, let's all trust him because some of his sperm became Thea.
After they cut ties, he kidnaps Ollie's kid, re-powers Damien Dahrk leading to Laurel's death, and drugs Thea into almost killing Ollie, but then they team up with him again for the final battle against Dahrk.
If he goes back to being a villain, I have no problem. Trying to make him an ally is dumb.