Well, for anyone who's having this kind of trouble, I've managed to solve it (not the best solution, but it's a start). For this, I'll be using cert-manager and letsencrypt.

First, I've created a ClusterIssuer to issue for my certs with letsencrypt:

apiVersion: certmanager.k8s.io/v1alpha1
kind: ClusterIssuer
metadata:      
  name: letsencrypt-prod-dns
spec:
  acme:
    dns01:
      providers:
      - azuredns:
          clientID: MY_AZURE_CLIENT_ID
          clientSecretSecretRef:
            key: client-secret
            name: azure-secret
          hostedZoneName: mydomain.com
          resourceGroupName: MY_AZURE_RESOURCE_GROUP_NAME
          subscriptionID: MY_AZURE_SUBSCRIPTION_ID
          tenantID: MY_AZURE_TENANT_ID
        name: azuredns
    email: somemail@mydomain.com
    privateKeySecretRef:
      key: ""
      name: letsencrypt-prod-dns
    server: https://acme-v02.api.letsencrypt.org/directory

Then I've created a fallback ingress to all my subdomains (this one will be the cert generator):

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  annotations:
    certmanager.k8s.io/acme-challenge-type: dns01
    certmanager.k8s.io/acme-dns01-provider: azuredns
    certmanager.k8s.io/cluster-issuer: letsencrypt-prod-dns
    ingress.kubernetes.io/force-ssl-redirect: "true"
    ingress.kubernetes.io/ssl-redirect: "true"    
    kubernetes.io/ingress.class: nginx    
  name: wildcard-ingress
  namespace: some-namespace  
spec:
  rules:
  - host: '*.mydomain.com'
    http:
      paths:
      - backend:
          serviceName: some-default-service
          servicePort: 80
        path: /      
  tls:
  - hosts:
    - '*.mydomain.com'
    - mydomain.com
    secretName: wildcard-mydomain-com-tls

Notice that I've declared at the TLS section the wildcard AND the absolute paths, so the cert will be valid for the URLs without subdomains too.

At this point, any requests to your domain, will be redirected to "some-default-service" with SSL(cert-manager will issue for a new cert as soon as you create the fallback ingress. This can take a while once cert-manager dns01 issuer is not mature yet), great!!!

But, what if you need to redirect some specific subdomain to another service? No problem (since they're running on the same namespace), all you have to do is to create a new ingress to your subdomain, pointing it to your existing wildcard-mydomain-com-tls cert secret:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  annotations:
    ingress.kubernetes.io/force-ssl-redirect: "false"
    ingress.kubernetes.io/ssl-redirect: "true"
    kubernetes.io/ingress.class: nginx
  name: somesubdomain-ingress
  namespace: some-namespace
spec:
  rules:
  - host: somesubdomain.mydomain.com
    http:
      paths:
      - backend:
          serviceName: some-other-service
          servicePort: 8080
        path: /        
  tls:
  - hosts:
    - somesubdomain.mydomain.com
    secretName: wildcard-mydomain-com-tls

Easy peasy lemon squeezy!!! Now your somesubdomain.mydomain.com overrides your fallback rule and sends the user to another app. The only thing you should notice here is that the secret is valid only for "some-namespace" namespace, if you need to use this cert in another namespace, you could:

1. Copy the secret from namespace "some-namespace" to "other-namespace". If you do this, remember that cert-manager will NOT renew this cert automatically for "other-namespace", so, you'd have to copy the secret again, every time your cert expires.
2. Recreate the fallback ingress to every namespace you have, so you'd have a new cert for each of them. This approach is more ingress verbose, but, it's fully automatic.

I guess that's it. Hope someone out there can benefit from this info.

Cheers