I disagree with the need to include promotional Mechs. They are crafted to be a very nicely designed/usable bonus for having supported AE via a purchase of AE-related merchandise. Of course, for some, it's the item the promo comes with that is the bonus. Regardless, they still do their job of being a gift for supporting AE. Making them NGs is pointless, as NGs can be obtained freely, thus defeating the purpose of them being bonuses for supporting AE directly.
As for designs, they seem decent thus far. What's lacking are more of a series. There's the traditional Soluna style, which is evident by the simple but machine-like appearance of the Hammerheads, the Newbatrons, the Volt Thorns, the Battle Hammers, the Katanas, and the Kurosawas, need sequel counterparts.
The SCMMs have their own special design theme (BC and Viking being exceptions); one that is only shared with the Skullcrusher, but they do their job of having a unique design that helps influence players to find means to get them. The House mechs also have their own special design theme. However, they're limited more by the lack of modern themed equipment than the lack of mech variety (though going back to an old suggestion of mine, it wouldn't hurt to have some sub-specializations of each base House mech (Advanced Series onwards)).
And mech designs themselves are not the main issue. The main issue is the lack of standard/mainstream designs in the same vein as the Katana or Battle Hammer for players to access w/o having to really work for it. For some that just hate a job or area too much to bother, having at least one decent mech that they can purchase without having to do anything for it (other than getting the money for it) does allow for some encouragement to continue playing; either to get the next "free" mech that looks even better, or in the hopes that a new and nicely designed "free" mech appears.