What Blizzard needs to do is completely ignore the dumb 'introduce units one by one' mechanic.
What, you mean you don't like the way they did it in Warcraft, Warcraft 2, Warcraft 3, and Starcraft?
Anyway having finally had a chance to play through it, I really don't get some of the complaints about the HOTS campaign. It's too easy? Since when were these games difficult? When you were new to the genre? I remember having some trouble with Starcraft and especially Brood Wars because I hadn't played a whole lot of other RTS games yet. In fact, I never could beat the last Zerg mission for Brood Wars. I played through them again shortly before SC2 came out and I just steamrolled through everything, even that last Zerg mission that had given me so much trouble in the past. In Warcraft 3 and TFT, only a couple of missions gave me trouble on hard, I steamrolled everything else. If you're a veteran of the RTS genre, you will rarely ever find much of a challenge in a single player campaign.
I do agree that they did a much better job introducing the new units like the Lurker and Corsair in Brood Wars. WoL wasn't quite so bad about it, but HotS really did seem like nearly every mission was pretty much just about using your new unlocked unit. The whole thing seem really phoned in. What I really hated was that most of the missions with a gimmick were just blatant fucking ripoffs of the same gimmick in WoL, like feeding the ancient being a direct copy of the WoL mission where you collected the fancy gas for Tosh. I also disliked how it seemed that 75% of the fucking campaign was timed missions.
Also could have done without the War3 shtick of having Kerrigan in every single fucking mission, since she's so absurdly powerful. The 'hero' missions in this one also sucked a lot of fucking ass compared to how fun they were in WoL, especially the mission to set charges in the worm tubes on Char. Also I hated the fact that Raynor changes his mind and shows up to help in the last mission. I actually liked how he was completely horrified that she had re-zergified and basically said 'get the fuck away from me', but then he's all like you know what fuck it.
I dunno, I've got super low expectations for the Protoss expansion. Probably gonna be more of the exact same, starting with Zeratul being judged by the rest of the Protoss for helping Kerrigan and then a campaign focused entirely around your unit unlocks, 90% timed missions, one or two gimmick missions that are a complete copy of gimmick missions from WoL/HotS, and then a final mission where Zeratul, Artanis, Valerian, Raynor, and Kerrigan all hold hands and dance in a circle around Amon to defeat him.