Two words: weapon balance.
Fusion wasn't TOO powerful in D1. It was hax, but that was the point. Halving the damage in D2 was a bad, bad decision and it sucked in D3 also.
If I were the one making the decisions, and were worried about Fusion being so overused that it overshone all the other weapons I wanted so desperately to showcase in the new game, I would decrease the damage by 25% rather than 50%, giving Fusion 45 rather than 30. That way it would be on par with the new weapons, rather than inferior.
I would also NOT nerf the Lasers to make way for the Super Lasers (which are weaker than the original Lasers from D2 anyway). That decision might have been thought through to some degree, but D2 introduced robots that were tougher, hit point-wise, than any robot from D1 - the Red Fatty Junior taking forever to die even from Level 6 Quads. I know that if the increment rate from D1 had been kept for the Super Lasers in D2, a Super Laser level 6 quad burst would do 60 damage rather than 50. Is that such a big deal, though? It's a bare 20% increase compared to as things are now, and a mere 15,38% increase compared to Descent 1's Level 4 Lasers. I, for one, never had the feeling that the D1 L4 Quads were overpowered.
As for Spreadfire - I also don't think its nerf was necessary in order to let Helix shine. Every D1 primary weapon, although generally inferior to its D2 counterpart, has something that makes you at least consider using it instead of the superior equivalent in SOME situations. In D2 Spread is just plain inferior to Helix and the only edge it has over it is manageable energy consumption. But you have weapons with similar energy consumption which are just plain better. Spread in D1 had stronger bolts (10 dmg compared to 9) and had among the fastest flying speeds of all projectiles (200 I believe, which was toned down to 120 in D2, if memory serves). Robot Spread as fired by the Fox robot has apparently kept the fast travelling speed, which is like a slap to the face. Spreadfire deserves better, as it really was a respectable weapon back in D1.
So now for a bit of shameless advertising - the HAM file which will come with the D1,5 mission in the distant future when it's been finished, fixes all of these problems. Ha.
-Same engine as D1 so the same engine change to tic based instead of frame based.
-Completely re-hash the secret level system so both D1 and D2 type secret levels can be made.
-In co-op make robots behave the same as in singleplayer (retroactively apply this to d1 if it needs to be, never tried co-op in d1)
-Nerf the hell out of the ice boss. Not so much the damage he does but the fact that you get blinded immediately upon entering the room.
-Smart blobs are kinda dumb. I'd get rid of them in favor of just giving a robot more hit points or resistance to specific weapons.
-Un-nerf the Spreadfire
-Un-castrate the Fusion
-Make phoenix bolts do less damage to the ship firing them, or scrap the phoenix completely and give us a new 4/b.
-Include the boss AI for the D1 bosses.
-D1 robots should have d1 sounds.
-Switches should be toggleable as well as one-shot.
-level briefings and better briefing music.
-more than 4 midi tracks.
Truer words have never been spoken. Great minds think alike, apparently! Haha.
I would pay a lot of money to be able to play a Descent 2 like this.
Also add the option to have in-game animated level exiting sequences like in D1 and the Interactive Demo, but that would require some graphics work (Quartzon and Baloris Prime would be particularly hard I think)