Is adding shards in the free DLCs possible?

I would imagine that it would be. The main thing would be in providing a way to access those new shards. Adding in new enemies for them to come from is possible, but extra work. So most likely anything added would be crafted. That way they can use as few new assets as possible, and just add code.

Although, I question if Resist Physical and Resist Ailment would be necessary. My thinking on the Resist Magic shard was that it existed to catch edge cases, where something wasn't one of the other elements.

Another shard I would add is a DIRECTIONAL ICE SHARD SERIOUSLY WHY ISN'T THERE ONE ALREADY IN THE GAME!?

I'm personally of the opinion that having some shards to work off of techniques would have been a good idea. The biggest downsides to techniques were their cost compared to their effect, and the ease of interrupting them. Having shards that boost the power, reduce the cost, or a shard that increases the speed of them with a chance to prevent flinching would all make a technique-focused run more viable. They wouldn't all have to be gold shards either. Imagine a red shard that lets you instant cast a technique if you do the input before it wears off, with the window getting larger as it powers up. Or how about a toggled blue shard where you enter a powered-up mode that drains MP over time, but while it's active techniques are stronger and have a high chance to not consume mana. A directional shard could act as a macro, activating different techniques based on the direction you aim, with the cost for an easier input being a higher cost.

Dunno what version you're playing but in mine, those resist shards exist. For instance, Kunekune drops the resist curse shard. I believe that malediction drops resist petrification and poison toad drops resist poison. The physical ones exist as well as far as I'm aware.