Let's be clear here: I'm not saying Tyranids should not be able to shoot. They have always been an army with some surprisingly effective ranged firepower and I want to see that continue. I also want to see that return to the old days when Tyranid firepower was effective, but unorthodox. However, what I don't want is for Tyranids to be limited to victory through shooting, which is pretty much what they are now, in a competitive or non-competitive environment.
armies that focus too strongly on close combat are quite boring to play against
Would you not say that the same is true of armies that focus too strongly on ranged firepower? I certainly would. I'm not against shooting stuff by any means (it's the only way I can get hard), but Tyranids shouldn't be an army dedicated to victory through shooting.
I'd also like to point out that you seem to believe that Tyranids are a potent close combat force at the moment. I would like to disabuse you of this notion. They suck at close combat right now against any army besides Tau.
What I don't want to see is Nids being able to take assault grenades or their equivalent.
This is lazy and frankly doesn't represent the idea that you might fend off each new monster as it climbs over the barricade, but sooner or later one of them is going to drag you down with it.
I'd love to know how you would try to make Tyranids (let's not just focus on the one race here people. I know I brought it up but Tyranids aren't the only race that have an issue with this, Chaos Daemons and even Eldar are in the same boat) effective in close combat without giving them assault grenades or equivalent. You could go back to the old Catalyst rules that let you strike back even after you die but that would turn every Tyranid combat unit into a suicide squad, so it would just replace one problem with a different one. It also wastes the high Initiative that most Tyranid combat specialists have, and pay a price for.
To be quite honest I think reinstating flesh hooks, or some similar option, is really the only viable way to get Tyranids back into an effective fighting force. When most of your combat specialists are less durable than an Eldar Guardian or an Ork boy, you pretty much have to strike first in combat, or at least have the chance to, in order to get things to work.
I'd be down for something like increased access to psychic powers across the army, and having the primaris power grant the effect of assault grenades (plus something else to make it more worthwhile), to increase the need for Tyranid units to rely on one another.