Khador: Marauder - I love this warjack for Khador and it's a shame he's not seen more often. As a reinforcement this guy shines for dealing with heavies. With one focus he can charge a heavy, another to boost to hit with his Combo Smite, and possibly even buy another attack if he needs to. It also goes without saying that since he's a Khador heavy, he's also pretty resilient and can take some damage. He's not so great at dealing with infantry, but if you can clear them out by other means, he can really dislodge an opposing heavy.
Cryx: Reaper - This guy often only sees play with Terminus, but with 3 free focus on a flank, this guy can cause some serious headaches. On the flank he can reach out to threaten some models with his typically short-ranged harpoon, boost to hit to drag them in, and then boost another hit with his helldriver to get the sustained attack, and you know the drill. While he doesn't fit the bill for survivability, Cryx as a faction doesn't usually have much in that department anyway, but they hit hard - and the Reaper does just that.
Menoth: Vengers - The Protectorate is one faction that I would say does not really benefit from having a warjack as reinforcements. Typically they want their warjacks close to their warcaster or choir, and as a reinforcement they would not be. Instead, Menoth should be using Vengers for some quick threats that are difficult to dislodge and can cause some headaches for infantry. Add to that the synergy that most Menoth warcasters have with units, and these guys are a solid choice without chewing up too much clock.
Retribution: Houseguard Halberdiers - Retribution is another faction that wouldn't really benefit from a heavy warjack on the flanks, but mostly because their warjacks are expensive and their warcasters don't usually offer much direct support. An argument could be made for the new Sphinx, but that would be specifically for Rahn. Halberdiers, however, offer a relatively threatening unit with reach that can also camp out on an objective in shield-wall formation just to be obnoxious with lots of bodies. One could also consider Destors for similar reasons to the Vengers above, but they tend to be more of a shooty-unit and I think reinforcements should have the goal of getting stuck-in.
Mercenaries: For the good ol' mercs the 7 point slot is a difficult one to fill for reinforcements. Rather, it's not difficult to find models to fill it, but rather difficult to choose which to use. Unfortunately, Mercenaries lack that wonderful 7-point warjack to fill this role, but fortunately they have a whole lot of solos instead. For Mercs I generally think that your choices will depend more heavily on what warcaster you will be bringing, as what might work for Magnus might not do so well for Ashlynn, etc. Some thoughts might be including one of the light warjacks and a solo or small units like Dannon Blythe & Bull, that you might not consider taking normally. You should also pay attention to any discounts you might receive in your tier list, as those also apply to your reinforcements!
Trollbloods: Long Riders - Trolls unfortuantely lack even an 8 point heavy warbeast, so 7 points is a little more challenging for this faction to fill. Fortunately, they have some useful cavalry models in the Long Riders. While Horthol makes them much better, even a min unit of these guys can be helpful for most Trollblood warlocks to utilize, especially since they can offer some great speed and pretty powerful attacks, especially thanks to their mounts. Don't forget that they get +2 to their charge attack rolls! Finally, they offer Trolls a quick response with some high SPD models that are self-sufficient - something that the typical Troll brick can certainly use.
Legion: Nephilim Bolt Thrower + Shepherd - Legion is such a beast-heavy faction that I'm surprised that they don't already have a 7-point warbeast. That said, they do have access to some amazing beasts and can drive those beasts to extreme levels because of their abilities to mitigate their fury. The bolt thrower and shepherd are a great pair for reinforcements because although you can normally force outside of your control area for reinforcement beasts, you might still have to deal with the load of fury that beast generates. This is where the shepherd comes in! Not only can that bolt thrower throw out a fully-boosted shot (hopefully triggering the critical) on the turn he comes in, but the shepherd can make all of that fury disappear! Having a shepherd on the flank will allow you to put on the pressure while allowing your main force to apply pressure elsewhere without worrying about keeping the bolt thrower in control.
I do have to add, however, that since Legion has a number of theme lists that offer discounts to warbeasts, it's also worth considering bringing a Seraph instead, if you get said discount.