template<class T1,class T2,class T3=t_dummy,class T4=t_dummy,class T5=t_dummy>
class t_aggregator:public T1,
                   public T2,
                   public T3,
                   public T4,
                   public T5
{};

Ошибка 1 error C2500: structure::t_aggregator<T1,T2>: 'structure::t_dummy' уже является прямым базовым классом    d:\Users\Dima\Work\lib\structure\t_aggregator.h    17

template<class T1,class T2>
class t_aggregator:public T1,
                   public T2
{};

template<class T1,class T2,class T3>
class t_aggregator:public T1,
                   public T2,
                   public T3
{};

Ошибка 1 error C2977: structure::t_aggregator: слишком много аргументов шаблон    d:\Users\Dima\Work\lib\structure\t_aggregator.h    23

template<size_t> struct t_dummy {};
template<class T1,class T2,class T3=t_dummy<3>,class T4=t_dummy<4>,class T5=t_dummy<5>>
class t_aggregator:public T1,
                   public T2,
                   public T3,
                   public T4,
                   public T5
{};

// T1, T2, T3, T4 - нужные базы

struct Your : T1, T2, T3, T4 { ..... }; // плоское наследование

struct tmp4 : T4 {};
struct tmp3 : T3, tmp4 {};
struct tmp2 : T2, tmp3 {};
struct tmp1 : T1, tmp1 {};
struct Your : tmp1 { ..... }; // последовательное наследование

#include <boost/mpl/inherit.hpp>

struct Your : boost::mpl::inherit< T1, T2, T3, T4 > { ..... };

class t_dummy {};

template<class T1,class T2,class T3=t_dummy,class T4=t_dummy,class T5=t_dummy>
class t_aggregator:public T1,
                   public T2,
                   public T3,
                   public T4,
                   public T5
{};

template<class T1,class T2,class T3,class T4>
class t_aggregator<T1, T2, T3, T4, t_dummy>:public T1,
                   public T2,
                   public T3,
                   public T4
{};

template<class T1,class T2,class T3>
class t_aggregator<T1, T2, T3, t_dummy, t_dummy>:public T1,
                   public T2,
                   public T3
{};


class C1 {};
class C2 {};

int main()
{
   t_aggregator<C1, C2> ta;
   return 0;
}