This would be a lot better with something like shower of sparks or lunge but as is doesn't seem well supported in draft since everyone can just splash the burn spells anyway. Maybe some more double red costing burn?

I believe the card doesn't work as well with those burn spells as you think. Each one would trigger Nexus twice, once for the player it damages, and once for the creature. You will still only get X damage for X mana. Do note that Static Discharge is akin to those cards you listed.

Also, I don't see this as being a very powerful card in limited due to the scarcity of burn. It's more of a johnny card for constructed.

it would allow that kind of multi-target burn the option of adding more damage to kill a bigger creature for example. Are you actually planning on making more sets and having a small constructed environment?

The eventual goal is to have a full standard format (two cores and two blocks). That will obviously take a while, however I do have the first set of the block to follow this fairly far in development.

Standard's only one core these days.

Well, it includes two core sets for three months, and then one for the other nine months.