Between shield cancelling and wavedashing, increasing the overall stun considerably may not really be needed if at all or even be a good idea to create some decent combos. The main problem for good comboing that would need to be addressed is the early
airdodging and A attacks the game lets you do before the stun actually finishes on tumble hits (I imagine it's after a certain part in the tumbling animation where it lets you do it).
http://www.youtube.com/watch?v=gsF_zFKyvkY
In the video you can see the attacks register as 'Consecutive Hits' even though it hits them
after they already started and nearly complete an airdodge. Assuming the consecutive hits counter goes off the stun like it did before, this gives you an idea how much stun there probably
would be if you couldn't just dodge or do an A attack in the middle of it anyway if it puts you into a tumble.
The few combos that do exist in the game already, like Sheik's f-tilt lock and such, generally involve weaker/stale hits that don't put you into a tumble, and weak hits that don't knock you down don't allow the early dodging and A attacks (again, the interrupt is linked to the tumble animation) so you actually remain fully stunned and unable to do
anything for the stun's entire duration like in the previous games.
Increasing the game's comboing ability could actually be
as simple as disabling that one thing, and not just increasing overall stun and then trying to balance it as well which could be a lot of work.
More importantly,
only increasing hitstun in general would likely just make the existing weak hit combos even better and create more of those dumb "lol-single-move-spam" type weak hit combos than anything else really, and is compounded by the fact that you can only A/SDI weak hits and can't DI them normally to change the direction you get sent like you can on tumble hits.