you could make all coastal provinces adjacent to the seas adjacent able to be moved to by an army.
(damn that sounds complex)
what I mean is that:
an army in London can move directly to the provinces of:
Wales, Brest, Picardy, Belgium, Holland, Denmark, Norway, Yorkshire & Edinburgh BUT has to be convoyed to provinces beyond the English Channel and the North Sea.
Likewise an army in Naples could move to Rome, Tuscany, Tunis, Greece & Albania without fleets (by virtue of the TYS & ION) but would need to be convoyed further. [Rome would only be able to move to Tuscany, Naples and Tunis].
Totally doable, and you can keep fleets as an active participant without resorting to ''airplanes'' etc...