Maybe next time, have it so that exp is universal (so that you have all your exp in one pool and you spend the same exp on all units). That way, the exp from maxed out units is still useful, and I don't find that any new units later on are useless.
i its a pretty good dame, but id like to have a little more unit variety, along with what people are saying. maybe, like, wide angle cannons, spread shot, snipers, etc.