I voted EU3 is complete, because my preferences for Paradox games are thus
1. Victoria 2
2. EU4
3. EU3 Exp
EU3 is nicely complete imo, yes things can be added but it's not like they'd be plugging any holes.
Though my preferences for an expansion are as following:
1. Parliaments,
even if they were just senates ported over from Rome:VV, it would be a great inclusion.
2. Internal policies,
a bit more depth then just sliders, things like law policy, tax policy, military policy, land reform, etc. It seems paradox is heading that way with HoI3's laws though, so see how it goes there, add some depth (different policies for different cultures, religions etc.)
3. Expanded Colonization options
Historically, colonization was handled mostly by private companies with very little governmental input. It's hard for me to say exactly what could be added in this area, but I would strongly urge there be a higher barrier to entry in terms of colonialism, I have a personal vendetta against european minors colonizing, most likely there would be a requisite policy and level of power before a nation got colonists (See Magna Mundi).
Also, I would suggest rethinking what it is to send a colonist. Sending a colonist should represent not 100 pilgrims going forth, but a royal expedition to found a base that pilgrims will then flock to based on other factors like religious persecution, economic hardship and war. Less colonists, more factors influencing natural colony growth.
4. More detailed demographics,
a hard one to do I know, but I found it both odd and contentious how each province was 100% one culture, and as Magna Mundi showed, people could never quite settle for whether Welsh was british or not, or whether this was that... but to change that would probably be too big for an exp and have to wait for EU4
5. Family trees
I always found the absence of royal politicking odd, and it was included in Rome:VV well enough.