Vortex was fun but I don't really miss them that much, and as you already mentioned they were unbalanced with the laser special shot being both really cheap AND powerful without upgrading the laser.  This can still be seen in medium 1 where you can hold down nearly the entire level with 4 un-upgraded lasers attached to the vortex and shock towers.  Attaching them to blasters doesn't really make sense since they aren't cheap (even level 1 is 125 mana) and you can only hit one creep with the special, and with missiles it's kind of hard to tell how much extra damage they are really doing, plus it takes so long to charge them up you can't rely on special shots as a strategy.  The shock-line feature was kind of cool, but you have to get a vortex to level 7 to use it, which is a waste of money in most levels, and it's not all that powerful.  The explosion feature was pretty worthless.

Ultimately I would probably favor a more standard "booster" tower, especially for swarm with it's hex grid.  Place 1 and everything 1 grid away gets a 10% boost, which increases 10% with each upgrade.  This would of course be priced appropriately expensive.  It might also be cool to have a strength booster and a speed (attack rate) booster.