Regarding "saving Chaos", I think it's a fallacy to think that Chaos Legion cards will be unwanted or not played after rotating out of Modern. When that happens, you can still play them in Wild. I play mostly in Wild myself and it's a lot of fun! I only have a Modern deck for playing (most of my older cards are on Land), but I can still climb quite high up the ranks. True, you probably need some of the older cards to compete at the Champion level, but if you're just a casual player like me then a Modern deck will still get you far in Wild.
That said, this Land Giant idea is a pretty cool concept. As a God of War fan, I especially like that you've picked names out of Norse mythology. :-) The numbers would need some tweaking to provide proper incentives (a 10% bonus feels way too low to me). I like having the bonus represented as & being granted by a new ability that all the Land Giant cards would have, rather than by the card itself. This opens the door to other cards having this ability in the future.
Instead of having the cards soulbound during the sale phase, it might be simpler to sell soulbound tokens that can later be used to mint the cards themselves once the sale phase concludes. This would be similar to how Runi minting worked, so perhaps some of that code code be re-used. That might also generate more excitement and anticipation, as people look forward to minting their cards on a certain date.
One thing that strikes me as a bit problematic, is that in order to use a Land Giant you'd have to forgo using another card with potentially higher base PP. So unless the bonus is much higher, or the base PP is higher, or the cost of minting a Land Giant is lower, you might not see as many people using these as you'd like. I have thousands of unused Land plots, and thousands of unopened CL packs. Would it really be beneficial for me to burn all those cards to mint Land Giants, as opposed to simply combining all my cards and directly putting them on plots? I suppose everyone would have to do this kind of number crunching for themselves to see if it's really worth it or not.
Also, the main thing that prevents me from expanding my Land empire for the time being is DEC. Lack of DEC means that I might not participate in this kind of promo event even if I wanted to, because I already have plenty of PP waiting to be used on Land. I just lack the DEC to stake on the plots... so it's not true that large land owners also have lots of DEC just waiting to be burned. I imagine large land owners mostly have all of their DEC staked on plots or in LPs rather than sitting around liquid. So that's also a trade-off: would the incentive be great enough for me to pull some DEC out of the LP and reduce my passive LP rewards in exchange for a bit of a land boost?
One alternative that might be interesting, is instead of minting a brand new card, you are allowed to choose an existing card which would have the Land Giant ability added to it. So you could give your current Land workers the bonus, which might reduce or avoid the necessity of having to do the kind of cost / benefit analysis mentioned above, and hence make it look like a more attractive option for current large land owners. Although it's probably more fun to have a brand new class of cards as you envision it.
From a dev point of view, no matter how you tweak it this is going to require a good bit of work and thus will need proper prioritization against existing projects. But I wouldn't view that as a reason not to do this; it's probably something that we can fit in at some point, but likely would need to be sometime next year rather than in the immediate future.
TL;DR: the idea is interesting and has potential, but needs to be massaged and polished a bit to really make it shine.