We can currently only forge resists for legacy towers (that’s what most of the community calls them at least). As the game progresses and evolves, the relevance for these “legacy towers” declines more and more. While some players still use legacy towers on their base, a vast majority of players use what has recently been put out: the howitzers, the pylons (not as popular, but players use them), the orrery, and the not-so-new, but still-ever-so-relevant flak towers.
All of the towers that take unique currencies to upgrade (so anything other than lumber) seem to pose much more of a threat to dragons of the current meta than any other tower, hence why I think elemental resists should be added to the forge. I am not going to suggest an introduction of Howitzer or Pylon resists, since those towers are still particularly new, and players seem to be prevailing against them without the resists.
Now, sure, there are dragons in the season tabs that you can get with sigils, and they have all different kinds of element-centric resists: dark, fire, electric, ice, and the cyclical Elemental Resist… and you will see the elemental resist appear quite often on lineage dragons starting in Harbinger… but there are some dragons, very good dragons, that don’t have a resist at all. Resists are paramount to the success of an attack, since they reduce damage taken by 70% (I believe). An immense amount of that damage is usually from an elemental tower—which is usually a flak (lightning towers play a huge part in it as well, but we already have resists for those, and you can forge them). Flaks are the main sources of damage simply because of how unpredictable they can be with their shots, so it would only make sense if the resists were made more accessible to the player base, to use on any dragon they’d like.
There could be options to forge Dark Flak resists, Fire Flak resists, Electro-Flak resists, Ice Flak Resists, and even the Elemental Resist itself. These resists would not be able to be applied to dragons that already have said resists, the same way certain resists and regular spells are restricted for certain dragons right now. The Elemental Resist would not be able to be applied to a dragon that already has the Elemental Resist. A dragon that has the Elemental Resist applied and has a primary resist that gives resistance to a specific tower automatically loses that resist, but the player cannot apply another spell to the dragon.
This would make attacks a lot easier to get through, since, again, flaks are the main and most-used damage-dealing towers. These resists appear quite frequently, but not on all dragons. Some of the best dragons are the dragons with no resist, and even though those dragons are so apparently good (since they can get through without a resist), a resist could save a player from failing the attack, should they make any mistakes while flying. This could all be implemented when the next forge level releases, whenever that will be.