TLDR:
Disneyland resort had two development routes to pick: Tokyo Disney Resort or Universal Orlando Resort and they seem to have mimicked the former.
There were several iterations of options bandied about. While the Toy Story lot does contain enough room for a gate in technicality, it would be smaller than current DCA and lack sufficient room for parking structures, hotels, amenities, external shopping and retail.
The Eastern Gateway new parking structure that will be nestled between Disney Way, Harbor and i5 is going to be needed for the current parks themselves. In fact it’s somewhat over-needed for the current parks. It couldn’t also support a third gate on the Toy Story lot. There is a world where garden walk was purchased by Disney, but they’ve turned down several opportunities over the years to buy that.
So that kind of presented them with two options, build a third gate or prioritize freeing up space for Disneyland and DCA major footprint expansions and have space left over for a concentrated resort/retail/dining complex next to the convention center.
Mayyyyybe a small water park with a resort complex and garage could fit I guess. Mayyybee. That’s completely speculative and they’ve made zero indications they want to do that.
But I think the moderate term plan is to continue expanding their current assets and even if they are super aggressive with that, it will occupy them for at least a couple more decades of ample expansion room.
In the far flung distant future if a third gate is extremely desired by the company, a Universal-like scenario where the next complex is more off site makes more sense. Because Toy Story is off site regardless. That would be the Angel stadium complex. That parcel with 50% more room seems minor, but that 50% room provides space for parking, dining, retail and accommodations. If it was their goal, which I don’t think it is.